 
1. The ultimate elements of which the human body is composed are azote, oxygen, and hydrogen (gaseous fluids); and carbon, phosphorus, calcium, sulphur, sodium, potassium, magnesium, and iron (solid substances). These bodies are called elementary and ultimate, because they are capable of being resolved by no known process into more simple substances. 2. These elementary bodies unite with each other in different proportions, and thus form compound substances. A certain proportion of azote uniting with a certain proportion of oxygen, hydrogen, and carbon, forms a compound substance possessing certain properties. Another proportion of azote uniting with a different proportion of oxygen, hydrogen, and carbon, forms another compound substance possessing properties different from the former. Oxygen, hydrogen, and carbon, uniting in still different proportions without any admixture 3. Each proximate principle constitutes a distinct form of animal matter, of which the most important are named gelatin, albumen, fibrin, oily or fatty matter, mucus, urea, pichromel, osmazome, resin, and sugar. 4. By chemical analysis it is ascertained that all the proximate principles of the body, however they may differ from each other in appearance and in properties, are composed of the same ultimate elements. Gelatin, for example, consists (in 100 parts) of azote 16-988/1000, oxygen 27-207/1000, hydrogen 7-914/1000, carbon 47-881/1000 parts. The elementary bodies uniting in the above proportions form an animal substance, soft, tremulous, solid, soluble in water, especially when heated, and on cooling, which may be considered as its distinctive property, separating from its solution in water into the same solid substance, without undergoing any change in its chemical constitution. 5. Again, albumen consists of azote 15-705/1000, oxygen 23-872/1000, hydrogen 7-540/1000, carbon 52-888/1000, parts. The elementary bodies uniting in these different proportions, there results a second proximate principle, an adhesive fluid, transparent, destitute of smell and taste, miscible in water, but 6. In like manner, fibrin consists of azote 19-934/1000, oxygen 19-685/1000, hydrogen 7-021/1000, carbon 53-360/1000 parts, forming a solid substance of a pale whitish colour and firm consistence, the peculiar character of which is its disposition to arrange itself into minute threads or fibres. 7. On the other hand, fat or oil, which is a fluid substance of a whitish yellow colour, inodorous, nearly insipid, unctuous, insoluble in water and burning with rapidity, consists of a larger proportion of hydrogen, a small proportion of oxygen, and a still smaller proportion of carbon, without any admixture of azote. 8. From this account of the composition of the proximate principles, which it is not necessary to extend further, it is manifest that all of them consist of the same ultimate elements, and that they derive their different properties from the different proportions in which their elements are combined. 9. The ultimate elements that compose the body are never found in a separate or gaseous 10. In like manner, the proximate principles never exist in a distinct and pure state, but each is combined with one or more of the others. No part consists wholly of pure albumen, gelatin, or mucus, but albumen is mixed with gelatin, or both with mucus. 11. Simple or combined, every proximate principle assumes the form either of a fluid or of a solid, and hence the most general and obvious division of the body is into fluids and solids. But the terms fluid and solid are relative, not positive; they merely express the fact that some of the substances in the body are soft and liquid compared with others which are fixed and hard; for there is no fluid, however thin, which does not hold in solution some solid matter, and no solid, however dense, which does not contain some fluid. 12. Fluids and solids are essentially the same in nature; they differ merely in their mode of aggregation; hence the easy and rapid transition from the one to the other which incessantly takes place in the living body, in which no fluid long remains a fluid, and no solid a solid, but the fluid is constantly passing into the solid and the solid into the fluid. 13. The relative proportion of the fluids in the human body is always much greater than that of the solids; hence its soft consistence and rounded 14. The fluids are not only more abundant than the solids, but they are also more important, as they afford the immediate material of the organization of the body; the media by which both its composition and its decomposition are effected. They bear nourishment to every part, and by them are carried out of the system its noxious and useless 15. The fluids, composed principally of water holding solid matter in solution, or in a state of mechanical division, either contribute to the formation of the blood, or constitute the blood, or are derived from the blood; and after having served some special office in a particular part of the system, are returned to the blood; and according to the nature and proportion of the substances they contain, are either aqueous, albuminous, mucous, gelatinous, fibrinous, oleaginous, resinous, or saline. 16. When the analysis of the different kinds of animal matter that enter into the composition of the body has been carried to its ultimate point, it appears to be resolvable into two primitive forms: first, a substance capable of coagulation, but possessing no determinate figure; and secondly, a substance having a determinate figure and consisting of rounded particles. The coagulable substance 17. When solid, the coagulable substance is disposed in one of two forms, either in that of minute threads or fibres, or in that of minute plates or laminÆ; hence every solid of the body is said to be either fibrous or laminated. The fibres or laminÆ are variously interwoven and interlaced, so as to form a net-work or mesh; and the interspaces between the fibres or laminÆ are commonly denominated areolÆ or cells (fig. XVII). 18. This concrete substance, fibrous or laminated, is variously modified either alone or in combination with the rounded particles. These different modifications and combinations constitute different kinds of organic substance. When so distinct as obviously to possess a peculiar structure and peculiar properties, each of these modifications is considered as a separate form of organized matter, and is called a PRIMARY TISSUE. Anatomists and physiologists have been at great pains to discriminate and classify these primary tissues; for it is found that when employed in the composition of the body, each preserves its peculiar structure and properties wherever placed, however combined, and to whatever purpose applied, undergoing 19. The first primary tissue is the peculiar substance termed MEMBRANE. It has been already stated (16) that one of the ultimate forms of animal matter is a coagulable substance, becoming concrete or solid under the process of coagulation. The commencement of organization seems to be the arrangement of this concrete matter into straight thready lines, at first so small as to be imperceptible to the naked eye. Vast numbers of these threads successively uniting, at length form a single thread of sufficient magnitude to be visible, but still smaller than the finest thread of the silkworm. If the length of these threads be greater than their breadth, they are called fibres; if, on the contrary, their breadth exceed their length, they are termed plates or laminÆ. By the approximation of these fibres or plates in every possible direction, and by their accumulation, combination, and condensation, is constituted the simplest form of organized substance, the primary tissue called membrane. 20. Membrane once formed is extensively employed 21. The properties which belong to membrane are cohesion, flexibility, extensibility, and elasticity. By its property of cohesion, the several parts of the body are held together; by its combined properties of cohesion, flexibility, and extensibility, the body in general is rendered strong, light, and yielding, while particular parts of it are made capable of free motion. But elasticity, that property by which parts removed from their situation in the necessary actions of life are restored to their natural position, may be regarded as its specific property. The varied purposes accomplished in the economy by the property of elasticity will be apparent as we advance in our subject. Meantime, it will suffice to observe that it is indispensable to the action of the artery in the function of the circulation; to the action of the thorax in the function of respiration; to the action of the joints in the function of locomotion: in a word, to the working of the entire mechanism by which motion of every kind and degree is effected. All these properties are physical, not vital; vital properties do belong even to this primary form of animal matter; but they are comparatively obscure. In the tissue with which organization commences, and which is the least removed from an inorganic substance, the properties that are prominent and essential are merely physical. 22. By chemical analysis, membrane is found to contain but a small proportion of azote, the 23. Membrane exists under several distinct forms; a knowledge of the peculiarities of which will materially assist us in understanding the composition of the body. The simplest form of membrane, and that which is conceived to constitute the original structure from which all the others art produced, is termed the cellular. When in thin slices, cellular membrane appears as a semi-transparent and colourless substance; when examined in thicker masses, Fig. XVII. A single film of the cellular tissue lifted up and slightly distended. 24. It is certain that the interspaces or cells of this membrane have no determinate form or size, that they communicate freely with each other, and that this communication extends over the whole body; for if a limb which has been infiltrated be frozen, a thousand small icicles will be formed, assuming the shape of the containing cells, some of which are found to be circular and others cylindrical, and so on. If air or water escape into any particular part of the body, it is often effused over the whole extent of it, and butchers are observed to inflate animals by making a puncture in some part where the cellular tissue is loose, and from 25. Cellular membrane, variously modified and disposed, forms the main bulk of all the other solid parts of the body, constituting their common envelope and bond of union, and filling up all their interstices. It is dense or loose, coarse or fine, according to its situation and office. Wherever it is subject to pressure, it is dense and firm, as in the palm of the hand and the sole of the foot; around the internal organs it is more loose and delicate, and it becomes finer and finer as it divides and subdivides, in order to envelope the soft and tender structures of the body. Fig. XVIII. A portion of cellular tissue, very highly magnified, showing the strings of globules of which its ultimate fibres are by some supposed to consist. 26. According to some who have carefully examined with the microscope its component threads, they consist of minute particles of a globular figure (fig. XVIII.); other microscopical observers regard 27. Every part of this tissue is penetrated by arteries, veins, absorbents, and nerves, endowing it with properties truly vital, though in a less degree than any of the other primary tissues; and varied and important as the uses are which it serves in the economy, the most manifest, though certainly not the only ones, are those which depend upon its physical properties of cohesion, flexibility, extensibility, and elasticity. Fig. XIX. 1, A portion of adipose tissue; 2, minute bags containing the fat; 3, a cluster of the bags, separated and suspended. 28. The tissue which contains the fat, termed the adipose, is the second form of membrane; it is obviously a modification of the cellular, from 29. The cells of the cellular tissue, as has been shown (24), are continuous over the whole body; but each adipose vesicle is a distinct bag, having no communication whatever with any other (fig. XIX. 2, 2). The cellular tissue is universally diffused; but the adipose is placed only in particular parts of the body; principally beneath the skin, and more especially between the skin and the abdominal muscles, and around some of the organs contained in the chest and abdomen, as the heart, the kidneys, the mesentery, and the omenta. In most of these situations some portion of it is generally found, whatever be the degree of leanness to which the body may be reduced; while in the cranium, the brain, the eye, the ear, the nose, and several other organs, there is none, whatever be the degree of corpulency. The uses of the fat, which are various, will be stated hereafter. 30. The third form of membrane is termed the serous. Like the adipose, serous membrane is a modification of the cellular, and, like it also, it is limited in its situation to particular parts of the body, that is, to its three great cavities, namely, the head, the chest, and the abdomen. To the two latter it affords an internal lining, and to all the organs contained in all the three cavities, it affords a covering. By its external surface it is united to the wall of the cavity or the substance of the organ it invests; by its internal surface it is free and unattached; whence this surface is in contact only with itself, forming a close cavity or shut sac, having no communication with the external air. Smooth and polished (fig. XX.), it is rendered moist by a fluid which is supposed to be exhaled in a gaseous state from the serum of the blood; and from this serous fluid the membrane derives its name. Fig. XX. A portion of intestine, showing its external surface or serous coat. 31. Though thin, serous membrane is dense, compact, and of great strength in proportion to its bulk: it is extensible and elastic; extensible, for it expands with the dilatation of the chest in inspiration; elastic, for it contracts with the diminished size of the chest in expiration. In like manner, it stretches with the enlargement of the stomach during a hearty meal, and contracts as the stomach gradually diminishes on emptying itself of its contents. It is furnished with no blood-vessels large enough to admit the colouring matter of the blood; but it is supplied with a great number of the colourless vessels termed exhalents, with the vessels termed absorbents, and with a few nerves. It indicates no vital properties, but those which are common to the simple form of the primary tissue. Its specific uses are to afford a lining to the internal cavities; to furnish a covering to the internal organs; by its polished and smooth surface, to allow a free motion of those organs on each other, and by the moisture with which it is lubricated, to prevent them from adhering together, however closely, or for however long a period they may be in contact. 32. The fourth form of membrane, the fibrous, named from the obvious arrangement of its component parts, consists of longitudinal fibres, large enough to be visible to the naked eye, placed parallel to each other, and closely united. Sometimes these fibres are combined in such a manner 33. The fifth form of membrane, the mucous (fig. XXI.), derives its name from the peculiar fluid with which its surface is covered, called mucus, and which is secreted by numerous minute glands, Fig. XXI A portion of the stomach, showing its internal surface or mucous coat. Unlike all the other tissues of this class, the mucous membranes are the immediate seat of some of the 34. The last form of membrane which it is necessary to our present purpose to particularize, is that which constitutes the external covering of the body, and which is called the skin. The skin is everywhere directly continuous with the mucous membranes that line the internal passages, and its structure is perfectly analogous. Both the external and the internal surface of the body may be said therefore to be covered by a continuous membrane, possessing essentially the same organization, and almost identically the same chemical composition. The skin is an organ which performs exceedingly varied and important functions in the economy, to the understanding of which it is necessary to have a clear conception of its structure; some further account of it will therefore be required; but this will be more advantageously given when the offices it serves are explained. Fig. XXII. Portions of cartilage, seen in section. 35. Such is the structure, and such are the properties, of the first distinct form of organized matter. The second primary tissue, termed the CARTILAGINOUS (fig. XXII.), is a substance intermediate Fig. XXIII. Membranous portion of bone; the osseous portion being so completely removed, that the bone is capable of being tied in a knot. 36. The third distinct form of organized matter is termed the OSSEOUS tissue. Bone is composed of two distinct substances, an animal and an earthy matter: the former organic, the latter inorganic. The animal or organic matter is analogous both in its nature and in its arrangement to cellular tissue; the earthy or inorganic matter consists of phosphoric acid combined with lime, forming phosphate of lime. The cellular tissue is aggregated into plates or laminÆ, which are placed one upon another, leaving between them interspaces or cells, in which is deposited the earthy matter (phosphate of lime). If a bone, for example, the bone called the radius, one of the bones of the fore-arm, be immersed in diluted sulphuric, nitric, muriatic, or acetic acid, it retains its original bulk Fig. XXIV. Earthy portion of bone. 37. In general, the osseous tissue is placed in the interior of the body. Even when bone approaches the surface, it is always covered by soft parts. It is supplied with but few blood-vessels, with still fewer nerves, with no absorbents large enough to be visible, so that though it be truly alive, yet its vital properties are not greatly developed. The arrangement of its component particles is highly curious; the structure, the disposition, and the connexion of individual bones afford striking examples of mechanism, and accomplish most important uses in the economy; but those 38. All the primary tissues which have now been considered consist of precisely the same proximate principles. Albumen is the basis of them all; with the albumen is always mixed more or less gelatin, together with a minute quantity of saline substance: to the osseous tissue is superadded a large proportion of earthy matter. With the exception of the mucous, the organization of all these tissues is simple; their vital properties are low in kind and in degree; their decided properties are physical, and the uses they serve in the economy are almost wholly mechanical. Fig. XXV. Portion of a muscle; showing (a) the muscular fibres and their parallel direction; and (b) the termination of the fibres in tendon. 39. But we next come to a tissue widely different in every one of those circumstances, a tissue consisting of a new kind of animal matter, and endowed with a property not only peculiar to itself, but proper to living substance, and characteristic of a high degree of vital power. Muscular tissue, the fourth distinct form of animal matter, commonly known under the name of flesh, is a substance resembling no other in nature. It consists of a soft and pulpy substance, having little cohesive power, arranged into fibres which are distinctly visible to the naked eye, and which are disposed in a regular and uniform manner, being placed close and parallel to each other (fig. XXV.). These fibres are every where pretty uniformly the same in shape, size, and general appearance, being delicate, soft, flattened, and though consisting only of a tender pulp, still solid (fig. XXV.). When examined under the microscope, fibres, which to the naked eye appear to be single threads, are seen to divide successively into smaller threads, the Fig. XXVI. Two portions of muscle; one of which, a, is covered with membrane; the other, b, is uncovered; c, the muscular fibres terminating in tendon. 40. The ultimate thread, or the minutest division of which the muscular fibre is susceptible, is called a filament; the smallest thread which can be distinguished by the naked eye is termed a Fig. XXVII. Portion of a muscle enclosed in a sheath of fascia or aponeurosis. 41. The composition of the ultimate filament has been very carefully examined by many distinguished physiologists with microscopes of high magnifying power. Under some of these microscopes the filament appears to consist of a series of rounded particles or globules of the same size as the particles of the blood when deprived of their colouring matter, so that it looks like a string of pearls (fig. XXVIII.), each globule being commonly stated to be about the 2000th part of an inch in diameter. But it is Fig. XXVIII. Ultimate fibres of muscle, very greatly magnified; showing the strings of globules of which they are supposed by some to consist. 42. With the exception of the organs of sense, the muscular tissue is more abundantly supplied with arteries, veins, and nerves, than any other substance of the body. Every ultimate thread or filament appears to be provided with the ultimate Fig. XXIX. The appearance of the ultimate muscular fibres and of their transverse lines, as seen under the microscope of Mr. Lister, when the object is magnified 500 diameters. 43. The proximate principle of which the muscular pulp is composed is fibrin. From the pulp, when inclosed in its sheath of membrane, albumen, jelly, various salts, and a peculiar animal extract called osmazome, are also obtained; but these substances are probably derived from the membranous, not the muscular, matter. Fibrin contains a larger proportion of azote, the element peculiar to the animal body, and by the possession of which its chemical composition is distinguished from that of the vegetable, than any other animal substance. Fig. XXX. Portion of the trunk of a nerve; dividing into branches. 44. Muscular tissue possesses a slight degree of Fig. XXXI. Ultimate fibres of nerve, very highly magnified; showing the strings of globules of which they consist. 45. The last primary tissue, termed the NERVOUS, is equally distinct in nature and peculiar in property. It consists of a soft and pulpy matter, of a brownish white colour (fig. XXX.). According to some, the nervous, like the muscular pulp, is composed of minute globules, arranged in the same manner like a string of pearls (fig. XXXI.); according to others, Fig. XXXII. Nervous fibres, deprived of their neurilema and unravelled, showing the smaller threads, or filaments, of which the fibres consist. 46. The nervous pulp is at least as liberally supplied with blood-vessels as the muscular; the vessels are spread out upon the nerve-coat, in which they divide into innumerable branches of extreme minuteness, the distribution of which is so perfect, that there is not a particle of nervous 47. Albumen, in conjunction with a peculiar fatty matter, constitutes the chief proximate principles of which the nervous tissue is composed. To these are added a small proportion of the animal substance termed osmazome, a minute quantity of phosphorus, some salts, and a very large proportion of water; for out of one hundred parts of nervous substance, water constitutes as much as eighty. Its peculiar vital property is sensibility; and as all motion depends on the contractility of the muscular fibre, so all sensation depends on the sensibility of the nervous substance. 48. Such are the primary tissues, or the several kinds of organized matter of which the body is composed; and from this account it is obvious that they consist of three only—namely, the concrete matter forming the basis of membrane, the pulpy matter forming the proper muscular substance, and the pulpy matter forming the proper nervous substance. Of these three kinds of animal matter the component parts of the body consist. In combining to form the different structures, these primary substances are intermixed and arranged in a great 49. As filaments unite to form fibres, and fibres to form tissues, so tissues unite to form organs: that is, bodies having a determinate size and figure, and capable of performing specific actions. The cellular, the muscular, and the nervous tissues are not organs; membranes, muscles, and nerves are organs. The tissue, the simple animal substance, is merely one of the elements of which the organ is composed; the organ is compounded of several of those simple substances, arranged in a determinate manner, and moulded into a given shape, and so constituting a specific instrument. The basis of the muscle is muscular tissue; but to this are added, invariably, membrane, often tendon, and always vessels and nerves. It is this combination that forms the specific instrument called a muscle, and that renders it capable of performing its specific action. And every such combination, with its appropriate endowment, constitutes an organ. 50. Organs are arranged into groups or classes, according as they possess an analogous structure, and perform an analogous function; and this assemblage constitutes a SYSTEM. All the muscles of the body, for example, whatever their size, form, situation, or use, have an analogous structure, and perform an analogous function, and hence are 51. An APPARATUS, on the contrary, is an assemblage of organs, it may be differing widely from each other in structure, and exercising various and even opposite functions; but all nevertheless concurring in the production of some common object. The apparatus of nutrition consists of the organs of mastication, deglutition, digestion, absorption, and assimilation. Among the individual organs which concur in carrying on these functions may be reckoned the lips, the teeth, the tongue, the muscles connected with the jaws, the gullet, the stomach, the duodenum, the small intestines, the pancreas, the liver, the lacteal vessels, the mesenteric glands, and the lungs. Many of these organs have no similarity in structure, and few have any thing analogous in function; yet all concur, each in its appropriate mode and measure, to the conversion of the aliment into blood. In the apparatus of respiration, in that of circulation, of secretion, of excretion; in the apparatus of locomotion, in the apparatus of sensation, and more especially in the apparatus of the specific sensations,—vision, 52. A clear idea may now be affixed to the terms structure and organization. Structure may be considered as synonymous with arrangement; the disposition of parts in a determinate order; that which is constructed or built up in a definite mode, according to a determinate plan. The arrangement of the threads of the cellular web into areolÆ or cells; the combination of the primary threads into fibres or laminÆ; the disposition of the muscular pulp into filaments, placed parallel to each other; the investment of the filaments in membraneous sheaths; the combination of the filaments, included in their sheaths, into fibres; the aggregation of fibres into fasciculi; and the analogous arrangement and combination of the nervous pulp, are examples of structure. But when those structures are applied to particular uses; when they are so combined and disposed as to form a peculiar instrument, endowed with a specific function; when the cellular fibres, for example, are so arranged as to make a thin, dense, and expanded tissue; when to this tissue are added blood-vessels, absorbents, and nerves; when, in a word, a membrane is constructed, an organ is formed; when, 53. The term organization is employed in reference both to the component parts of the body, and to the body considered as a whole. We speak of an organized substance and of an organized body. An organized substance is one in which there is not only a definite arrangement of its component parts (structure), but in which the particular arrangement is such as to fit it for accomplishing some special use. Every organized substance is therefore essentially a special organ; limited in its object it may be, and perhaps only conducive to some further object; but still its distinctive character is, that it has a peculiar structure, fitting it for the accomplishment of some appropriate purpose. On the other hand, an organized body is a congeries of organs—the aggregate of the individual organs. Attention was directed in the early part of this work to one peculiar and essential character, by which such an organized 54. From the whole, then, we see that the human body is a congeries of organs; that those organs are constructed of a few simple tissues; and that all its parts, numerous, diversified, and complex as they are, are composed of but three primary forms of animal matter variously modified and combined. 55. But though by the analysis of its component parts, this machine, so complex in its construction, and so wonderfully endowed, may be reduced to this state of simplicity; and although this analytical view of it be highly useful in enabling us to form a clear conception of the nature of its composition; yet it is only by considering its individual parts such as they actually are, and by studying their situation, connexion, structure, and action, that 56. Viewing then the human body as a complicated whole, as a congeries of organs made up of various combinations of simple tissues, it may be observed, in reference to its external configuration, that it is rounded. This rounded form is principally owing to the large proportion of fluids which enter into its composition. The roundness of the face, limbs, and entire surface of the child, are in striking contrast to the unequal and irregular surface of the old man, whose humours are comparatively very much smaller in quantity. 57. The length of the human body exceeds its breadth and thickness; the degree of the excess varying at different periods of life, and according to the peculiar constitution of the individual. In the extremities, the bones, muscles, vessels, and nerves, are especially distinguished by their length. Fig. XXXIV. Front view of the skeleton. 1. the head; 2. the trunk; 3. the superior extremities; 4. the inferior extremities. Fig. XXXV. Back view of the skeleton. 1. the head; 2. the trunk; 3. the superior extremities; 4. the inferior extremities. 58. The form of the human body is symmetrical, that is, it is capable of being divided into two lateral and corresponding halves. Suppose a median line to pass from the vertex of the head through the centre of the spinal column (fig. XXXIV. 1, 2); if the body be well formed, it will be divided by this line into two exactly equal and corresponding portions (fig. XXXV. 1). This symmetrical disposition of the body is not confined to its external configuration. It is true of many of the internal organs; but principally, as has been already stated, of those that belong to the animal life. The brain and the spinal cord are divisible into two exactly equal halves (figs. XLVIII. d, and XLIX. 1, 2, 3); the organs of sense are double and symmetrical: the muscles of one side of the body exactly correspond to those of the other (fig. XXXIII.); the two hands and arms and the two lower extremities are alike (figs. XXXIV., XXXV.); but for the most part, the organs of the organic life, the stomach, the intestines, the liver, the spleen, for example, are single, and not symmetrical. 59. The human body is divided into three great portions, the head, the trunk, and the extremities (figs. XXXIV. and XXXV. 1, 2, 3, 4). 60. By the head is meant all that part of the body which is placed above the first bone of the neck (fig. XXXIV. 1). It is of a spheroidal figure, broader and deeper behind than before, somewhat like an egg in shape, with the broad end behind; it is flattened at its sides (figs. XXXV. 1, and XXXVI. 2, 4). Its peculiar figure renders it at once stronger and more capacious than it could have been had it possessed any other form. It is supported by its base on the spinal column, to which it is attached by the peculiar structure termed a joint (fig. XXXIV.), and fastened by ligaments of exceeding strength. 61. The head contains the central organ of the nervous system; the organs of the senses, with the exception of that of touch; and the organs of Fig. XXXVI. 1. Frontal bone; 2. parietal bone; 3. occipital bone; 4. temporal bone; 5. nasal bone; 6. malar bone; 7. superior maxillary bone; 8. inferior maxillary bone. Fig. XXXVII. Bones of the skull, separated; front view. 1. Frontal bone; 2. portions of the parietal bones; 3. malar or cheek bones; 4. nasal bones; 5. superior maxillary or bones of the upper jaw; 6. the vomer; 7. the inferior maxillary or bone of the lower jaw. Fig. XXXVIII. Bones of the skull separated; side view. 1. Frontal bone; 2. parietal bone; 3. occipital bone; 4. temporal bone; 5. nasal bone; 6. malar bone; 7. superior maxillary bone; 8. the unguis; 9. the inferior maxillary bone. Fig. XXXIX. Bones forming the base of the skull; viewed from the inside. 1. Occipital bone; 2. temporal bones; 3. sphenoid bone; 4. ethmoid bone; 5. superior maxillary bones, or bones of the upper jaw; 6. malar or cheek bones; 7. foramen magnum. 62. The bones of the cranium are eight in number, six of which are proper to the cranium, and two are common to it and to the face. The six bones proper to the cranium are the frontal (fig. XXXVII. 1), the two parietal (fig. XXXVI. 2), the two temporal (fig. XXXVIII. 4), and the occipital (fig. XXXVIII. 3); the two common to the cranium and face are the ethmoidal (fig. XXXIX. 4), and the sphenoidal (fig. XXXIX.3). The frontal bone forms the entire forepart of the vault (fig. XXXVII. 1); the two parietal form the upper and middle part of it (fig. XXXVIII. 2); the two temporal form the lower part of the sides (fig. XXXVIII. 4); the occipital forms the whole hinder part, together with a portion of the base (figs. XXXVIII. 3, XXXVI. 3, XXXIX. 1); while the ethmoidal forms the forepart, and the sphenoidal the middle part of the base (fig. XXXIX. 3, 4). Fig. XL. Portions of the bones of the cranium; showing the corresponding inequalities in their margins: which margins, when in apposition, constitute the mode of union termed suture. 1. External surface of the bone; 2. internal surface. Fig. XLI and Fig. XLII. 1. Side view of the adult skull, showing the several bones united by suture; 2. side view of the foetal skull, showing the bones imperfectly ossified, separated to some extent from each other, the interspace being occupied by membrane. The small size of the face compared with that of the cranium is strikingly apparent. 63. These bones are firmly united together. The union of bones is technically called an articulation or joint. All joints are either immoveable or moveable. The union of the bones of the cranium affords an example of an immoveable articulation. Prominences and indentations, like the teeth of a saw, are formed in the margins of the contiguous bones (figs. XXXVIII. and XL.). At these inequalities of surface, which are exactly adapted to each other (figs. XXXVIII. and XL.), the two bones are in immediate apposition in such a manner as to preclude the possibility of motion, and even to render the separation extremely difficult. This mode of articulation is termed a suture. There are certain advantages in constructing the cranium of several distinct bones, and in uniting Fig. XLIII. Section of the skull. 1. Cavity of the cranium occupied by the brain; 2. cut edge of the bones of the cranium, showing the two tables of compact bone and the intervening spongy texture called diploË. 64. The vault of the cranium forms a cavity which contains the brain (fig. XLIII.and XLVIII.) The size of this cavity is invariably proportioned to that of the organ it lodges and protects. The form and magnitude of the cavity, and consequently the shape and size of the cranium, depend upon the brain, and not of the brain upon the cranium. The soft parts model and adapt to themselves the hard, and not the hard the soft. The formation of the brain in the foetus is anterior to that of the case which ultimately contains it; and the hard bone is moulded upon the soft pulp, not the pulp upon the bone. At every period of life, on the 65. Tracing them from without inwards we see, then, that the various coverings afforded to the brain, the central organ of the animal life, seated in its vaulted cavity, are: 1. The tegument, consisting of the skin and of cellular and adipose membrane. 2. Beneath the tegument, muscles, in the forepart and at the vertex, comparatively slender and delicate; at the sides and posteriorly, 66. The bones of the face consist of fourteen, namely, the two superior maxillary or jaw-bones 67. Besides the bones and the teguments, the face contains a number of muscles, which for the most part are small and delicate (fig. XLIV.), together with a considerable portion of adipose matter; while, as has been stated, the face and head together contain all the senses, with the exception of that of touch, which is diffused, more or less, over the entire surface of the body. Fig. XLIV. Muscles of the face. 68. The second great division of the body, termed the TRUNK, extends from the first bone of the neck to that called the pubis in front, and to the lower end of the coccyx behind (fig. XXXIV. 2). It is subdivided into the thorax, the abdomen, and the pelvis (fig. XLV.). 69. The thorax or chest extends above from the first bone of the neck, by which it is connected with the head, to the diaphragm below, by which it is divided from the abdomen (figs. XLV. and 70. The figure of the thorax is that of a cone, the apex being above (fig. XLV.), through the aperture of which pass the tubes that lead to the lungs and stomach, and the great blood-vessels that go to and from the heart (fig. LX.). The base of the cone is slanting, and is considerably shorter before than behind, like an oblique section of the cone (fig. XLV.). 71. The osseous portion of the walls of the thorax is formed behind by the spinal column, a range of bones common indeed to all the divisions of the trunk; for it constitutes alike the posterior boundary of the thorax, abdomen, and pelvis (fig. 72. From above downwards, that is, from the first bone of the neck to the first bone of the sacrum, the separate bones forming the column progressively increase in size; for this column is the chief support of the weight of the head and trunk, and this weight is progressively augmenting to this point (fig. XLV. 2, 4). From the sacrum to the coccyx, the bones successively diminish in size, until, at the extremity of the coccyx, they come to a point (fig. XXXV.). The spinal column may therefore be said to consist of two pyramids united at their base (fig. XLV. 4, 5). The superior pyramid is equal in length to about one third of the height of the body, and it is this portion of the column only that is moveable. 73. The two surfaces of the spinal column, the anterior and the posterior, present a striking contrast (figs. XXXIV. and XXXV.). The anterior 74. From the posterior surface of the column, which is every where irregular and rough, spring, along the median line, in regular series, strong, sharp, and pointed projections of bone (fig. XXXV.), which from being sharp and pointed, like elongated spines, are called spinous processes, and have given name to the whole chain of bones. These processes afford fixed points for the action of powerful muscles. Extending the whole length of the column, from the base of the skull to the sacrum, on each side of the spinous processes, are deep excavations, which are filled up with the powerful muscles that maintain the trunk of the body erect. 75. From the lateral surfaces of the column likewise spring short but strong projections of bone, termed transverse processes, which also give attachment to powerful muscles (fig. XLVI.). Fig. XLV. Bones of the trunk. 1. Spinal column; 2. the seven cervical vertebrÆ; 3. the twelve dorsal vertebrÆ; 4. the five lumbar vertebrÆ; 5. the sacrum; 6. the sternum; 7. the true ribs; 8. the false ribs; 9. the clavicle; 10. the scapula; 11. the ilium; 12. the ischium; 13. the pubes; 14. the acetabulum; 15. the brim of the pelvis. 76. The separate bones of the series have a kind of turning motion on each other; hence each is called a vertebra, and the name of vertebral column is often given to the entire series, as well as 77. By inspecting fig. XLVI. 1, it will be seen that the upper and under edges of each vertebra consist of a ring of bone, of a firm and compact texture, rendering what may be called the body of the vertebra exceedingly strong (fig. XLVI. 3). This ring of bone forms a superficial depression (fig. XLVI. 2), for the reception of a peculiar substance, immediately to be described, which is interposed between each vertebra (fig. XLVII. 2). 78. The anterior surface of the body of the vertebra is convex (fig. XLVI. 3); its posterior surface is concave (fig. XLVI. 4); from the posterior surface springs a bony arch (figs. XLVI. 5 and LIII. 1), which, together with the posterior concavity, forms an aperture of considerable magnitude (fig. XLVI. 6), a portion of the canal for the passage of the spinal cord (figs. XLVII. 3, and XLIX. 3). Fig. XLVI. View of some of the vertebrÆ, which by their union form the spinal column. a. A vertebra of the neck; b. a vertebra of the back; a vertebra of the loins. 1. Ring of compact bone forming, 3, the body of the vertebra; 2. superficial depression for the reception of the intervertebral cartilage; 3. anterior surface of the body of the vertebra; 4. posterior surface; 5. bony arch; 6. opening for the passage of the spinal cord; 7. opening for the passage of the spinal nerves; 8. articulating processes by which the vertebrÆ are joined to each other; 9. two dorsal vertebrÆ united, showing the arrangement of, 10, the spinous processes; 11. a portion of a rib articulated with the side of the vertebra. 79. Both the upper and under edges of the arch form a notch (fig. XLVI. 7.), which, together with a corresponding notch in the contiguous vertebra, completes another aperture rounder and smaller than the former, but still of considerable size (fig. XLVI. 7.), the passage of the spinal nerves (fig. XLVII. 3). 80. From both the upper and under sides of the arch proceed two short but strong projections of bone (fig. XLVI. 8.), termed the articulating processes, because it is chiefly by these processes that the vertebrÆ are connected together. From the beginning to the end of the series, the two upper processes of the one vertebra are united with the two lower processes of the vertebra immediately above it (fig. XLVI. 9), and around the edges of all the articulating processes are visible rough lines, which mark the places to which the articulating ligaments are attached. 81. No vertebra, except the first, rests immediately upon its contiguous vertebra (fig. XLV. 2, 4). Each is separated from its fellow by a substance of a peculiar nature interposed between them, termed the intervertebral substance (figs. Fig. XLVII. 1. One of the Lumbar vertebrÆ. 2. Intervertebral substance. 3. A portion of the spinal cord in its canal. 82. The column is not constructed in such a manner as to admit of an equal degree of motion in every part of it. Every thing is contrived to give to that portion which belongs to the neck freedom of motion, and, on the contrary, to render that portion which belongs to the back comparatively fixed. In the neck the mechanism of the articulating processes is such as to admit of an equal degree of sliding motion forwards, backwards, and from side to side, together with a turning motion of one bone upon another; at the same time, the intervertebral substance between the several vertebrÆ is thick. In consequence of this mechanism, we can touch the breast with the chin, the back with the hind head, and the shoulders with the ear, while we can make the head describe more than a semicircle. But, in the back, the articulating processes are so connected as to prevent the possibility of any motion, either forwards or backwards, or any turning of one vertebra upon another, while the intervertebral substance is comparatively 83. The canal for the spinal cord, formed partly by the concavity in the posterior surface of the vertebra, and partly by the arch that springs from it (fig. XLVI. 6.), is lined by a continuation of the dense and strong membrane that constitutes the internal periosteum of the cranium, the dura mater (fig. XLVIII. c), which, passing out of the opening in the occipital bone, called the foramen magnum (figs. XXXIX. 7, and XLIX. 3), affords a smooth covering to the canal throughout its whole extent. Fig. XLVIII a. The scalp, turned down. b. The cut edge of the bones of the skull. c. The external strong membrane of the brain (Dura Mater) suspended by a hook. d. The left hemisphere of the brain, showing its convolutions. e. The superior edge of the right hemisphere. f. The fissure between the two hemispheres. Fig. XLIX 1. Hemispheres of the brain proper, or cerebrum; 2. hemispheres of the smaller brain, or cerebellum; 3. spinal cord continuous with the brain, and the spinal nerves proceeding from it on each side. 84. The spinal cord itself, continuous with the substance of the brain, passes also out of the cranium through the foramen magnum into the spinal canal (fig. XLIX. 3), enveloped in the delicate membranes that cover it, and surrounded by the aqueous fluid contained between those membranes. The size of the spinal canal, accurately adapted to that of the spinal cord, which it lodges and protects, is of considerable size, and of a triangular shape in its cervical portion (fig. XLIX. 3), smaller and rounded in its dorsal portion (fig. XLIX. 3), and again large and triangular in its lumbar portion (fig. XLIX. 3). 85. The spinal column performs several different, and apparently incompatible, offices. First, it affords a support and buttress to other bones. It sustains the head (fig. XXXIV. 1); it is a buttress to the ribs (fig. XLVI. 7); through the sternum and ribs it is also a buttress to the superior, and through the pelvis, to the lower, extremities (fig. XXXIV. 2, 3, 4). Secondly, it affords a support to powerful muscles, partly to those that maintain the trunk of the body in the erect posture against the force of gravitation, and partly to those that act upon the superior and inferior extremities in the varied, energetic, and sometimes long-continued movements they execute. Thirdly, it forms one of the boundaries of the great cavities that contain the chief organs of the organic life. To the support and protection of those organs it is specially adapted; hence the surface in immediate contact with them is even and smooth; hence its different curvatures, convexities, and concavities, have all reference to their accommodation; hence in the neck it is convex (fig. XLV. 2), in order to afford a firm support to the esophagus, the wind-pipe, the aorta, and the great trunks of the venous system (fig. LX. 3, 4); in the back it is concave, in order to enlarge the space Fourthly, it forms the osseous walls of a canal (figs. XLVI. 6, and XLVII. 3) for the lodgment and protection of the soft and tender substance of the spinal cord, one of the great central masses of the nervous system, the seat of the animal life (fig. XLIX. 3). Fifthly, it affords in its osseous walls secure apertures for the passage of the spinal nerves (figs. XLVI. 7, and XLIX. 3), by which impressions are transmitted from the organs to the spinal cord and brain, in the function of sensation; and from the spinal cord and brain to the organs in the function of volition. 86. For the due performance of these offices, it is indispensable that it should be firm, rigid, strong, and yet to a certain extent readily flexible in every direction. By what mechanism is it endowed with these apparently incompatible properties? 87. By means of the ring of compact bone, which forms so large a part of its body (fig. XLVI. 1) it is rendered firm, rigid, and strong. By means of its numerous separate pieces, exactly adjusted to 88. But the flexibility required is much greater than could be obtained by this expedient alone. A rigid and immoveable pile of bones, in the position of the spinal column, on which all the other parts of the body rest, and to which they are directly or indirectly attached, would necessarily have rendered all its movements stiff and mechanical; and every movement of every kind impossible, but in a given direction. That the movements of the body may be easy, free, and varied; that it may be possible to bring into play new and complex combinations of motion at any instant, with the rapidity of the changes of thought, at the command of the impulses of feeling, it is indispensable that the spinal column be flexible in every direction, forwards, backwards, and at the sides: it is equally indispensable that it be thus capable of yielding, without injuring the spinal cord; without injuring the spinal nerves; without injuring the thoracic and abdominal viscera; and without injuring the muscles of the trunk and extremities. The degree in which it possesses this power of flexibility, and the extent to which, by the cultivation 89. But the flexibility acquired through the combined properties of compressibility and elasticity is exceedingly increased by the action of the pulpy and inelastic matter in the centre of the intervertebral substance; this matter serving as a pivot to the vertebrÆ, facilitating their motion on each 90. But besides securing freedom and extent of motion, the intervertebral substance serves still another purpose, which well deserves attention. Firmness and strength are indispensable to the fundamental offices performed by the column; and to endow it with these properties, we have seen that the external concentric layers of the intervertebral substance are exceedingly tough and that 91. Alike admirable is the mechanism by which the separate pieces of the column are joined together. If but one of the bones were to slip off its corresponding bone, or to be displaced in any degree, incurable paralysis, followed ultimately by death, or instantaneous death, would happen; for pressure on the spinal cord in a certain part of Fig. L. 1. Common anterior ligament; 2. intervertebral substance. The anterior ligament is removed to exhibit (3.) the crucial fibres passing over it. Fig. LI. 1. Portion of the occipital bone; 2. common anterior ligament. 92. Corresponding with the ligament on the Fig. LII. 1. Posterior vertebral ligament. 93. Moreover, the bony arches of the vertebrÆ (fig. LIII. 1) are connected by means of a substance Fig. LIII. 1. Arches of the vertebrÆ seen from within; 2. ligaments connecting them. 94. And in the last place, the articular processes form so many distinct joints, each being furnished with all the apparatus of a moveable joint, and thus possessing the ordinary provision for the articulation of bones, in addition to the whole of the foregoing securities. 95. "In the most extensive motion of which the spinal column is capable, that of flexion, the common anterior ligament is relaxed; the fore part of the intervertebral substance is compressed, and its back part stretched; while the common posterior ligament is in a state of extension. In the extension 96. "The number and breadth of the attachments of these bones," says an accomplished anatomist and surgeon, 97. So the object of the construction of the spinal column being to combine extent and freedom of motion with strength, and it being necessary to the accomplishment of this object to build up the column of separate pieces of bone, the connecting substances by which the different bones are united are constituted and disposed in such a manner as to prove absolutely stronger than the bones themselves. Such is the structure of this important portion of the human body considered as a piece of mere mechanism; but our conception of its beauty and perfection would be most inadequate if we did not bear in mind, that while the spinal column performs offices so varied and apparently so incompatible, it forms an integrant portion of a living machine: it is itself alive: every instant, 98. The anterior boundary of the thorax is formed by the bone called the sternum, or the breast-bone, which is broad and thick at its upper, and thin and elongated at its lower extremity (figs. XLV. 6, and LIV.), where it gives attachment to a cartilaginous appendix, which being pointed and somewhat like a broadsword, is called the ensiform cartilage. 99. Its position is oblique, being near the vertebral column at the top, and distant from it at the bottom (fig. XLV. 6). Its margins are thick, and marked by seven depressions, for the reception of the cartilages of the seven true ribs (fig. LIV). Its anterior surface is immediately subjacent to the skin, and gives attachment to powerful muscles, which act on the superior extremities: its posterior surface is slightly hollowed in order to enlarge the cavity of the thorax (fig. LV.). Fig. LV. Posterior view of the sternum. 100. The thorax is bounded at the sides by the ribs, which extend like so many arches between 101. The ribs successively and considerably increase in length as far as the seventh, by which the cavity they encompass is enlarged; from the seventh they successively diminish in length, and the capacity of the corresponding part of the cavity is lessened. The direction of the ribs from above downwards is oblique (fig. XLV. 7, 8). Their external or anterior surface is convex (fig. XLV. 7, 8); their internal or posterior surface is concave: by the first their strength is increased; by the second the general cavity of the thorax is enlarged (fig. XLV. 7, 8). Their upper margin is smooth and rounded, and gives attachment to a double layer of muscles, called the intercostal, placed in the intervals that separate the ribs from each other (fig. LIX.). Along the lower margin is excavated a deep groove, for the lodgment and protection of the intercostal vessels. 102. The ribs are connected with the spinal column chiefly by what is termed the anterior ligament (fig. LVI. 1), which is attached to the Fig. LVI. Ligaments connecting the ribs to the spinal column. 1. anterior ligaments; 2. interarticular ligament; 3. ligaments of the necks of the ribs. The cartilages of the seven superior ribs are attached to the sternum by a double layer of ligamentous Fig. LVII. 1, &c. Ligaments connecting the ribs to the vertebrÆ behind. While thus firmly attached to their points of support, the ligaments, which fix them, are so disposed as to render the ribs capable of being readily moved upwards and downwards: upwards in inspiration; downwards in expiration; and it is by this alternate action that they enlarge and diminish the cavity of the thorax in the function of respiration. Fig. LVIII. Ligaments joining the cartilages of the ribs to the sternum. 103. Such are the boundaries of the cavity of the thorax as far as its walls are solid. The interspaces between these solid portions at the sides are Fig. LIX. A view of the muscles called Intercostals, filling up the spaces between the ribs. 104. The inner surface of the walls of the thorax, in its whole extent, is lined by a serous membrane, exceedingly thin and delicate, but still firm, called the pleura. The same membrane is reflected over 105. A fold of each pleura passes directly across the central part of the cavity of the thorax; extending from the spinal column to the sternum, and dividing the general cavity into two. This portion of the pleura is called the mediastinum, from its situation in the centre of the thorax, and it so completely divides the thoracic cavity into two, that the organs on one side of the chest have no communication with those of the other; so that there may be extensive disease in one cavity (for example, a large accumulation of water,) while the other may be perfectly sound. 106. The main organs contained in the cavity of the thorax are the lungs with their air tube; the heart with its great vessels; and the tube passing from the mouth to the stomach (fig. LX.). 107. The two lungs occupy the sides of the chest (fig. LX. 5). They are completely separated from each other by the membranous partition just described, the mediastinum. Between the two 108. The lungs are moulded to the cavities they fill; whence their figure is conical, the base of the cone being downwards, resting on the diaphragm (fig. LX. 5, b); and the apex upwards, towards the neck (fig. LX. 5). 109. That surface of each lung which corresponds to the walls of the chest is convex in its whole extent (fig. LX. 5); on the contrary, that surface which corresponds to the mediastinum is flattened (fig. LX. 5). The basis of the lung is concave, adapted to the convexity of the diaphragm on which it rests (fig. LX. 5). 110. The air-vessel of the lungs, termed the bronchus, together with the blood-vessels and nerves, enter the organ at its flattened side, not exactly in the middle, but rather towards the upper and back part. This portion is termed the root of the lung. 111. The lungs are attached to the neck by the trachea (fig. LX. 4), the continuation of which forms the bronchus; to the spinal column by the pleura, and to the heart by the pulmonary vessels (fig. LX. 3, d): their remaining portion is free and unattached. 112. In the living body, the lungs on each side completely fill the cavity of the chest, following passively the movements of its walls, and accurately adapting themselves to its size, whether its capacity enlarge in inspiration, or diminish in expiration, so that the external surface of the lung (the pulmonary pleura) is always in immediate contact with the lining membrane of the walls of the cavity (the costal pleura); consequently, during life, there is no cavity, the chest being always completely full. Fig. LX. a. The cut edges of the ribs, forming the lateral boundaries of the cavity of the thorax. b. The diaphragm, forming the inferior boundary of the thorax, and the division between the thorax and the abdomen. c. The cut edges of the abdominal muscles, turned aside, exposing the general cavity of the abdomen. 1. The cut edge of the pericardium turned aside. 2. The heart. 3. The great vessels in immediate connexion with the heart. 4. The trachea, or wind-pipe. 5. The lungs. 6. The liver. 7. The stomach. 8. The large intestine. 9. The small intestines. 10. The urinary bladder. 113. The anterior surface of the pericardium, the bag which envelopes the heart, lies immediately behind the sternum, and the cartilages of the second, third, fourth, and fifth ribs, covered at its sides by the pleura, and firmly attached below to the diaphragm (fig. LX. 1). 114. Surrounded by its pericardium, within the mediastinum, the heart is placed nearly in the centre of the chest, but its direction is somewhat oblique, its apex being directly opposite to the interval between the fifth and sixth ribs on the left side (fig. LX. 2); while its basis is directed upwards, backwards, and towards the right (fig. LX. 2). That portion of its surface which is presented to view on opening the pericardium is convex (fig. LX. 2); but its opposite surface, namely, that which rests upon the part of the pericardium which is attached to the diaphragm, is flattened (fig. LX. 1). It is fixed in its situation partly by the pericardium and partly by the great vessels that go to and from it. But under the different states of expiration and inspiration, it 115. The second division of the trunk, the abdomen, is bounded above by the diaphragm (fig. LXI. 2), below by the pelvis (fig. LXI. 3), behind and at the sides by the vertebrÆ and muscles of the loins (fig. LXIII.), and before by the abdominal muscles (fig. LXIII. 9). 116. The organ which forms the superior boundary of the abdomen, the diaphragm (midriff), is a circular muscle, placed transversely across the trunk, nearly at its centre (fig. LXI. 2). It forms a vaulted partition between the thorax and the abdomen (fig. LXI. 2). All around its border it is fleshy (fig. LXI. 2); towards its centre it is tendinous (fig. LXI. 2); the surface towards the abdomen is concave (fig. LXI. 2); that towards the thorax convex (fig. LXI. 2); while its middle tendinous portion ascends into the thorax as high as the fourth rib (fig. LXI. 2). Fig. LXI. View of the diaphragm. 1. Cavity of the thorax; 2. diaphragm separating the cavity of the thorax from that of the abdomen; 3. cavity of the pelvis. 117. The diaphragm is perforated by several apertures, for the transmission of tubes and vessels, which pass reciprocally between the thorax and abdomen (fig. LXII.). 1. A separate aperture is formed to afford an exit from the thorax of the aorta, the common source of the arteries (fig. LXII. 2), and an entrance into the thorax of the thoracic duct, the tube that bears the digested aliment to the heart. 2. A little to the left of the former, there is another aperture, through which passes the esophagus or gullet (fig. LXII. 3), the tube that conveys the food from the mouth to the stomach. 3. On the right side, in the tendinous portion of the diaphragm, very carefully constructed, is a third aperture for the passage of the vena cava (fig. LXII. 4), the great vessel that returns the blood to the heart from the lower parts of the body. Fig. LXII. View of the diaphragm with the tubes that pass through it. 1. Arch of the diaphragm; 2. the trunk of the aorta passing from the chest into the abdomen; 3. the esophagus passing from the chest through the diaphragm to the stomach; 4. the vena cava, the great vein that returns the blood to the heart from the lower parts of the body, passing from the abdomen, into the chest, in its way to the right side of the heart; 5. 6. muscles that arise in the interior of the trunk and that act upon the thigh; 5. the muscle called psoas; 6. the muscle called iliacus. 118. The partition formed by the diaphragm between the thorax and abdomen, though complete, is moveable; for as the diaphragm descends in inspiration and ascends in expiration, it proportionally enlarges or diminishes the cavities between which it is placed; consequently, the actual magnitude of these cavities varies every moment, and the size of the one is always in the inverse ratio of that of the other. 119. Between the abdomen and the pelvis there is no separation; one cavity is directly continuous with the other (fig. LXI. 3); but along the inner surface of the expanded bones, which form a part of the lateral boundary of the abdomen, there is a prominent line, termed the brim of the pelvis (fig. XLV. 15), marking the point at which the abdomen is supposed to terminate and the pelvis to commence. 120. Behind and at the sides the walls of the abdomen are completed partly by the lumbar portion of the spinal column and partly by the lumbar muscles (fig. XLV. 4), and before by the abdominal muscles (fig. LXIII. 9). 121. The inner surface of the walls of the abdomen is lined throughout by a serous membrane, termed the peritoneum (fig. LXIII.). From the 122. Like the thorax, the abdomen is always completely full. When the diaphragm is in action, it contracts. When the diaphragm is in the state of contraction, the abdominal and lumbar muscles are in the state of relaxation. By the contraction of the diaphragm, the organs contained in the abdomen are pushed downwards, and the anterior and lateral walls of the cavity being at this moment in a state of relaxation, they readily yield, and, consequently, the viscera are protruded forwards and at the sides. But the abdominal and lumbar muscles in their turn contract, the diaphragm relaxing; and, consequently, the viscera, 123. The contents of the abdomen consist of the organs which belong to the apparatus of digestion, and of those which belong to the apparatus of excretion. 124. The organs which belong to the apparatus of digestion are—1. The stomach (fig. LXIII. 2) 2. The duodenum (fig. LXIII. 4). 3. The jejunum (fig. LXIII. 5). 4. The ilium (fig. LXIII. 5). The three last organs are called the small intestines, and their office is partly to carry on the digestion of the aliment commenced in the stomach, and partly to afford an extended surface for the absorption of the nutriment as it is prepared from the aliment. 5. The pancreas (fig. LXIV. 5). 6. The liver (fig. LXIV. 2). 7. The spleen (fig. LXIV. 4). The three last organs truly belong to the apparatus of digestion, and their office is to co-operate with the stomach and the small intestines in the conversion of the aliment into nutriment. Fig. LXIII. 1. Esophagus; 2. stomach; 3. liver raised, showing its under surface; 4. duodenum; 5. small intestines; 6. cÆcum; 7. colon; 8. urinary bladder; 9. gall bladder; 10. abdominal muscles divided and reflected. 125. The organs which belong to the apparatus of excretion are—1. The large intestines consisting Fig. LXIV. General view of the viscera of the abdomen. 1. Stomach raised; 2. under surface of liver; 3. gall bladder; 4. spleen; 5. pancreas; 6. kidneys; 7. ureters; 8. urinary bladder; 9. portion of the intestine called duodenum; 10. portion of the intestine called rectum; 11. the aorta. 126. The last division of the trunk, called the pelvis (fig. LXI. 3), consists of a circle of large and firm bones, interposed between the lower portion of the trunk and the inferior extremities (fig. XLV.). The bones that compose the circle, distinct in the child, are firmly united in the adult into a single piece; but as the original separation between each remains manifest, they are always described as separate bones. They are the sacrum (fig. XLV. 5), the coccyx (fig. XXXV.), the ilium (fig. XLV. 11), the ischium (fig. XLV. 12), and the pubis (fig. XLV. 13). 127. The sacrum, placed like a wedge between the moveable portion of the spinal column and the lower extremities, forms the posterior boundary of the pelvis. The figure of this bone is triangular (fig. XLV. 5); its anterior surface is concave and smooth, for enlarging the cavity of the pelvis and sustaining the organs contained in it (fig. XLV. 5); its posterior surface is convex, 128. The base or upper part of the sacrum receives the last vertebra of the loins on a large and broad surface (fig. XLV. 4), forming a moveable joint; and the degree of motion at this point is greater than it is at the higher points of the spinal column. Firmly united at its sides with the haunch bones, it admits there of no degree of motion. 129. The coccyx, so named from its resemblance to the beak of the cuckoo, when elongated by a succession of additional bones, forms the tail in quadrupeds; but in man it is turned inwards to support the parts contained in the pelvis, and to contract the lower opening of the cavity. By means of a layer of cartilage, the medium by which this bone is connected with the sacrum, it forms a moveable articulation, continuing moveable in men until the age of twenty-five, and in women until the age of forty-five; continuing moveable in women thus long, in order that by yielding to the force which tends to push it backwards during the period of labour, it may enlarge the lower aperture of the pelvis, and so facilitate the process of parturition and diminish its suffering. 130. The lateral boundaries of the pelvis are 131. The ischium or hip bone is the lower part of the pelvis (fig. XLV. 12); at its undermost portion is a rounded prominence called the tuberosity (fig. XLV. 12), in its natural condition covered with cartilage, upon which is superimposed a cushion of fat. It is this part on which the body is supported in a sitting posture. 132. The pubis or share bone forms the upper and fore part of the pelvis (fig. XLV. 13), and together with the two former bones, completes the large and deep socket, termed the acetabulum (fig. XLV. 14), into which is received the head of the thigh-bone (fig. XXXIV. 4). The margin of the acetabulum and the greater part of its internal surface is lined with cartilage, so that in its natural condition it is much deeper than it appears to be when the bones alone remain. 133. The lower aperture of the pelvis, which appears large when all the soft parts are removed, is not really large, for in its natural state it is filled up partly by muscles and partly by ligaments, which sustain and protect the pelvic organs, leaving only just space enough for the passage to and from those which have their opening on the external surface. 134. The cavity of the pelvis, together with all the organs contained in it, are lined by a continuation of the membrane that invests the abdomen and its contents. 135. The organs contained in the pelvis are the rectum (fig. LXIV. 9), which is merely the termination of the large intestines, the urinary bladder (fig. LXIV. 8), and the internal part of the apparatus of reproduction. 136. The large and strong bones of the pelvis not only afford lodgment and protection to the tender organs contained in its cavity, but sustain the entire weight of the body, the trunk resting on the sacrum as on a solid basis (fig. XLV. 5), and the lower extremities being supported in the sockets in which the heads of the thigh-bones play, in the varied movements of locomotion (fig. XXXIV. 4). 137. The last division of the body comprehends the superior and the inferior extremities. 138. The superior extremities consist of the shoulder, arm, fore-arm, and hand. 139. The soft parts of the SHOULDER are composed chiefly of muscles; its bones are two, the scapula or the blade bone, and the clavicle or the collar bone (fig. LXV. 2, 4). Fig. LXV. 1. Sternum; 2. clavicle; 3. ribs; 4. anterior surface of scapula; 5. coracoid process of scapula; 6. acromion process of scapula; 7. margin of glenoid cavity of scapula; 8. body of the humerus or bone of the arm; 9. head of the humerus. 140. The SCAPULA is placed upon the upper and back part of the thorax, and occupies the space from the second to the seventh ribs (fig. LXV. 4) Fig. LXVI. 1. Posterior surface of scapula; 2. margin of scapula; 3. acromion process; 4. margin of glenoid cavity; 5. clavicle; 6. body of humerus; 7. head of humerus. Unlike that of any other bone of the body, it is embedded in muscles, without being attached to any bone of the trunk, excepting at a single point. From the bones of the thorax it is separated by a double layer of muscles, on which it is placed as upon a cushion, and over the smooth surface of which it glides. Originally, like the bones of the skull, it consisted of two tables of compact bone, with an intermediate layer of spongy bony substance (diploË); but, by the pressure of the muscles that act upon it, it gradually grows thinner and thinner, until, as age advances, it becomes 141. The figure of the scapula is that of an irregular triangle (fig. LXVI.). Its anterior surface is concave (fig. LXV. 4), corresponding to the convexity of the ribs (fig. XLV. 7); its posterior surface is very irregular (fig. LXVI. 1), being in some parts concave and in others convex, giving origin especially to two large processes (figs. LXV. 5, and LXVI. 3); one of which is termed the acromion (fig. LXVI. 3), and the other the coracoid process of the scapula (fig. LXV. 5). The margins of the bone, whatever the thinness of some portions of it, are always comparatively thick and strong (fig. LXVI. 2), affording points of origin or of insertion to powerful muscles. At what is called the anterior angle of the bone there is a shallow oval depression covered with cartilage and deepened by a cartilaginous margin, called the glenoid cavity of the scapula (figs. LXV. 7, and LXVI. 4), which receives the head of the humerus or bone of the arm (figs. LXV. 9, and LXVI. 7, 6). 142. The clavicle, the second bone of the shoulder, is a long and slender bone, of the form of an italic , projecting a little forwards towards its middle, so as to give a slight convexity of outline to the top of the chest and the bottom of the neck (fig. LXV. 2). It is attached by one extremity to the sternum (fig. LXV. 2) and by the other to 143. In the joint formed by the clavicle and the sternum (fig. LXVII. a) an interarticular cartilage is placed between the two bones which are united, first by a strong fibrous ligament, which envelops them as in a capsule (fig. LXVII. 1); by a second ligament, which extends from the cartilage of the first rib to the clavicle (fig. LXVII. 4), by which the attachment of the clavicle to the sternum is materially strengthened; and by a third ligament which passes transversely from the head of one clavicle to that of the other (fig. LXVII. 3). The joint thus formed, though so strong and firm that the dislocation of it is exceedingly Fig. LXVII. 1. The fibrous capsule of the sternum and clavicle; 2. the same laid open, showing a, the interarticular cartilage; 3. the ligament connecting the two clavicles; 4. the ligament joining the clavicle to the first rib; 5. ligaments passing down in front of the sternum. 144. The scapular extremity of the clavicle (fig. LXVIII. 6) is attached to the processes of the scapula (fig. LXVIII. 4. 3) by several ligaments of great strength (fig. LXVIII. 7, 8, 9). First by very strong fasciculi which pass from the upper surface of the clavicle to the acromion of the scapula (fig. LXVIII. 6); and secondly by two ligaments Fig. LXVIII. 1. The clavicle; 2. the anterior part of the scapula; 3. the coracoid process; 4. the acromion process; 5. the humerus; 6. ligaments binding the scapular end of the clavicle to the acromion; 7. 8. 9. ligaments passing from one process of the scapula to the other; 10. the fibrous capsule of the shoulder-joint. 145. The clavicle serves the following uses: it sustains the upper extremity; it connects the upper extremity with the thorax; it prevents the upper extremity from falling forwards upon the thorax; and it affords a fixed point for steadying the extremity in the performance of its various actions. 146. The glenoid cavity of the scapula (fig. LXV. 7) receives the head of the humerus, the bone of the arm (fig. LXV. 9), and the two bones being united by ligament form the shoulder-joint (fig. LXVIII.). This joint is what is termed a ball and socket joint, the peculiarities of which are two: first, beyond all others this mode of articulation admits of free and extensive motion; in the present case, there is the utmost freedom of motion in every direction, upwards, downwards, backwards, and forwards. In the second place, this mode of articulation admits of the motion of the limb without that of the body, or of the motion of the body 147. It does not seem possible to construct a joint of great strength, capable, at the same time, of the degree of motion possessed by the joint of the shoulder. So shallow is the socket of the scapula, and so large the head of the humerus, that it seems as if the slightest movement must dislodge it from its cavity (fig. LXVI. 4. 7). For sustaining heavy weights or resisting a great amount of pressure, applied to it suddenly and in various directions, the arm is obviously unfitted. But this is not its office. The superior extremities are the organs of apprehension—the instruments by which the mind executes the commands of the will. They do not need the strength required by the organs that sustain the weight of the body and that perform the function of locomotion; but they do need freedom and extent of motion: to this strength may be sacrificed, and so it is; yet what can be done to combine strength with mobility is effected. Large and strong processes of bone, proceeding 148. The arm consists of numerous and powerful 149. The upper end of the humerus terminates in a circular head (fig. LXV. 9), which is received into the socket of the scapula (fig. LXV. 9. 7) Fig. LXIX. 1. Lower extremity of the humerus; 2. grooved surface; 3. internal condyle; 4. external condyle; 5. the upper part of the ulna; 6. the head; 7. the neck; 8. the tubercle of the radius. 150. The muscles that act upon the arm arise from the back (fig. LXXII. 2), the chest (fig. LXXI. 1), the clavicle (fig. LXXI. 1), and the scapula (fig. LXXI. 3); and they move the arm with freedom and power upwards, downwards, forwards, backwards, inwards, and outwards. The chief muscle that raises the arm is the deltoid (fig. LXXI. 3), which arises partly from the clavicle and partly from the scapula (fig. LXXI. 3). It has the appearance of three muscles proceeding in different directions, the different portions being separated by slight fissures (figs. LXXI. 3, and LXXII. 3). The fibres converging unite and form a powerful muscle which covers the joint of the humerus (fig. LXXI. 3). It is implanted by a short and strong tendon into the middle of the humerus (fig. LXXI. 4). Its manifest action is to pull the arm directly upwards. Its action is assisted by the muscles that cover the back of the scapula, which are in like manner inserted into the humerus, and Fig. LXXI. View of the muscles on the fore part of the chest that act upon the arm. 1. The muscle called the great pectoral; 2. the small pectoral; 3. the deltoid; 4. the humerus. 151. The principal muscle that carries the arm downwards is the latissimus dorsi (fig. LXXII. 2), the broadest muscle of the body, which, after having covered all the lower part of the back and loins, terminates in a thin but strong tendon which stretches to the arm, and is implanted into the humerus (fig. LXXII. 2), near the tendon of a muscle immediately to be described,—the great pectoral. When the arm is raised by the deltoid and its assistant muscles, the latissimus dorsi carries it downwards with force, and its powerful action is increased by that of other muscles which arise from the scapula and are inserted into the arm. 152. The principal muscle that carries the arm forwards towards the chest, is the great pectoral (fig. LXXI. 1), which, arising partly from the clavicle (fig. LXXI. 1), partly from the sternum (fig. LXXI. 1), and partly from the cartilages of the second, third, fourth, fifth, and sixth ribs (fig. LXXI. 1), covers the greater part of the breast (fig. LXXI. 1). Its fibres, converging, terminate in a strong tendon, which is inserted near the tendon of the longissimus dorsi into the humerus, about four inches below its head (fig. LXXI. 1). These two muscles form the axilla or armpit, the anterior border of the axilla consisting of the pectoral muscle. Though each of these muscles has its own peculiar office, yet they often act in concert, thereby greatly increasing their power, and the result of their combined action is to carry the arm either directly downwards or to the side of the chest. Fig. LXXII. View of the muscles seated on the back part of the trunk that act upon the shoulder and arm. 1. The muscle called the trapezius; 2. the latissimus dorsi; 3. the deltoid. 153. Some of the muscles that elevate the arm carry it inwards, and others outwards; the muscles that carry it forwards likewise carry it inwards; while of the muscles that pull it downwards, some direct it forwards and inwards, and others backwards and outwards (151 and 152). 154. It has been already stated that the shoulder-joint is completely surrounded by the muscular fibres or the tendinous expansions of several of these powerful muscles, which have a far greater effect in maintaining the head of the humerus in its socket than the fibrous capsule of the joint; the latter being necessarily loose, in order to allow of the extended and varied motions of the arm, whereas the muscles that encompass the joint adhere closely and firmly to it. Moreover, by virtue of their vital power these muscles act with an efficiency which a mere ligamentous band is incapable of exerting; for they apportion the strength of resistance to the separating force, and react with an energy proportioned to the violence applied. 155. The bones of the fore-arm are two, the ulna and the radius (figs. LXIX. and LXXIII.). The ulna is essentially the bone of the elbow (figs. LXIX. 5, and LXXIII. 3); the radius that of the Fig. LXXIII. 1. The internal condyle of the humerus; 2. the external condyle of the humerus; 3. the olecranon process of the ulna; 4. the head of the radius. 156. The upper end of the ulna belonging to the elbow is large (figs. LXIX. 5, and LXXIII. 3). It sends backwards the large projection commonly named the elbow or olecranon (fig. LXXII. 3), 157. The radius, the second bone of the fore-arm, placed along its outer part next the thumb, is small at its upper end (figs. LXIX. 6, and LXXIII. 4); but its body is larger than that of the ulna; while its lower end, next the wrist to which it properly belongs, is very bulky (fig. LXXXII. 1). Its upper end is formed into a small circular head, which is united by distinct joints both to the humerus and to the ulna (fig. LXIX. 6). The top of its rounded head is excavated into a shallow cup (figs. LXIX. 6, and LXXIII. 4) which receives a corresponding convexity of the 158. The joint of the elbow is composed above of the condyles of the humerus (fig. LXIX. 3. 2), and below by the heads of the ulna and radius (fig. LXIX. 5. 6). 159. The upper surface of the ulna is so accurately adapted to the lower surface of the humerus that the one seems to be moulded on the other (figs. LXIX. 5, and LXXIII. 3), and the form of these corresponding surfaces, which are everywhere covered with cartilage, is such as to admit of free motion backwards and forwards, that is, of extension and flexion; but to prevent any degree of motion in any other direction. The joint is therefore a hinge-joint, of which the two motions of flexion and extension are the proper motions. This hinge is formed on the part of the humerus by a grooved surface, with lateral projections (fig. LXIX. 2, 3, 4), and on the part of the ulna by a middle projection with lateral depressions (fig. LXIX. 5): the middle projection of the ulna turning readily on the grooved surface of the humerus (fig. LXIX. 2). 160. The bones are held in their proper situation, first, by a ligament on the fore part of the arm, called the anterior (fig. LXXIV. 6), which arises from the lower extremity of the humerus, and is inserted into the upper part of the ulna and the coronary ligament of the radius (fig. LXXIV. 6. 161. At their inferior extremities the ulna and radius are united partly by the interosseous ligament (fig. LXXVII. 1) and partly by ligamentous fibres which pass transversely from one bone to the other (fig. LXXVII. 2) on the anterior and the posterior surface of the fore-arm. Fig. LXXVII. 1. Interosseous ligament; 2. transverse fibres passing between the radius and ulna, and uniting the two bones; 3. 4. 5. posterior and lateral ligaments of the wrist joint; 6. ligaments uniting the bones of the wrist with one another; 7. 8. ligaments which attach the metacarpal to the carpal bones; 9. transverse ligaments for the attachment of the phalanges of the fingers; 10. lateral ligaments for the attachment of the phalanges of the fingers 11. ligaments of the thumb. 162. The lower extremity of the radius is also united to the wrist; and the hand being attached to the wrist, the junction of the hand and the fore-arm is effected by the articulation of the wrist with the radius (fig. LXXVII.). The ligaments which 163. The muscles that act upon the fore-arm are placed upon the arm (fig. LXXVIII.). The joint of the elbow being a hinge-joint, the fore-arm can admit only of two motions, namely, flexion and extension. The muscles by which these motions are effected are four, two for each; the two flexors being placed on the fore part (fig. LXXVIII. 2. 4), and the two extensors on the back part of the arm (fig. LXXIX. 5). 164. The two flexor muscles of the fore-arm are termed the biceps and the brachialis (fig. LXXVIII. 2, 4). The biceps is so called because it has two distinct heads or points of origin (fig. LXXVIII. 2), both of which arise from the scapula (fig. LXXVIII. 2). About a third part down the humerus the two heads meet, unite and form a bulky muscle (fig. LXXVIII. 2), which, when it contracts, may be felt like a firm ball on the fore part of the arm, the upper part of the ball marking the point of union of the two heads (fig. LXXVIII. 2). The muscle gradually becoming smaller, at length terminates in a rounded tendon (fig. LXXVIII. 3), which is implanted into the tubercle of the radius a little below its neck (fig. LXXVIII. 3). It is an exceedingly thick and powerful muscle, and its manifest action is to bend the fore-arm with great strength. But since its tendon is inserted into the radius, besides bending the fore-arm, it assists other muscles that also act upon the radius in the performance of a function to be described immediately (168). Fig. LXXVIII. View of the flexor muscles of the fore-arm. 1. The anterior surface of the scapula; 2. the muscle called biceps; 3. tendon of the biceps passing to the tubercle of the radius; 4. the muscle called brachialis. 165. The second flexor of the fore-arm, termed the brachialis, is placed immediately under the biceps, and is concealed by it for a considerable part of its course (fig. LXXVIII. 4). Arising from the humerus, on each side of the insertion of the deltoid, it continues its attachment to the bone all the way down the fore part of the humerus, to within inch of the joint; it then passes over the joint, adhering firmly to the anterior ligament (fig. LXXVIII. 4), and is inserted by a strong tendon into the ulna (fig. LXXVIII. 4). It is a thick and fleshy muscle, powerfully assisting the action of the biceps. 166. The two extensor muscles are named the triceps and the anconeous (fig. LXXIX.). The triceps, seated on the back part of the arm, derives its name from having three distinct points of origin, or three separate heads (fig. LXXIX. 5); one of which arises from the scapula and two from the humerus (fig. LXXIX. 5). All these heads adhere firmly to the humerus, as the brachialis does on the fore part of the arm, down to within an inch of the joint (fig. LXXIX. 5), where they form a strong tendon, which is implanted into the olecranon of the ulna (fig. LXXIX. 3); the projection of which affords a lever for increasing the action of the muscle. In all animals that leap and bound, this process of the ulna is increased in length in proportion to their power of performing these movements. The triceps forms an exceedingly thick and strong muscle, which envelops the whole of the back part of the arm (fig. LXXIX.); its action is simple and obvious; it powerfully extends the fore-arm. The anconeous, a small muscle of a triangular form, arising from the external condyle of the humerus, and inserted into the ulna a little below the olecranon, assists the action of the triceps. Fig. LXXIX. View of the extensor muscles of the fore-arm. 1. The scapula; 2. the upper part of the humerus; 3. upper end of the ulna; 4. upper end of the radius; 5. the muscle called triceps, the extensor of the fore-arm. 167. Such are the motive powers which act upon the fore-arm, and which produce all the motions of which the hinge-joint of the elbow renders it capable. But besides flexion and extension, the fore-arm is capable of the motion of rotation, which is accomplished by means of the radius. It has been shown (157) that the top of the rounded head of the radius is excavated into a shallow cup (figs. LXIX. 6, and LXXIII. 4) which receives a corresponding convexity of the humerus (figs. LXIX. 2, and LXXIII. 2). In consequence of this articulation with the humerus, the radius, like the ulna, can move backwards and forwards in flexion and extension, the proper movements of the hinge-joint; but that portion of the margin of the hinge of the radius which is in apposition with the ulna is convex (fig. LXIX. 6), and is received into a semilunar cavity hollowed out in the ulna (fig. LXIX. 5). In this cavity the rounded head of the radius revolves, the two bones being held together by the ligament 168. The principal supinators arise from the external condyle of the humerus (fig. LXXX.), and are called long and short (fig. LXXX. 4, 5). The long supinator extends as far as the lower end of the radius, into which it is inserted (fig. LXXX. 4): the short supinator surrounds the upper part of the radius, and is attached to it in this situation (fig. LXXX. 5.). Moreover, the triceps, being inserted into the radius (164), often cooperates with the supinators and powerfully assists their action. 169. The principal pronators are also two, called the round and the square (figs. LXXXI. and LXXXVI. 1). The round pronator arises from the internal condyle, and passing downwards, is inserted into the middle of the radius (fig. LXXXI. 4); the square pronator is a small muscle between the radius and ulna, at their lower extremities being attached to each (fig. LXXXVI. 1). Fig. LXXX. View of the supinators of the radius and hand. 1. The humerus; 2. the ulna; 3. the radius; 4. the muscle called the long supinator passing to be inserted into the lower portion of the radius; 5. the muscle, called the short supinator, surrounding the upper part of the radius. 170. The action of these muscles in producing the rotation of the radius, and so rendering the hand supine or prone, is sufficiently manifest from the mere inspection of the diagrams (fig. LXXXI. 4). Fig. LXXXI. View of the pronators of the hand. 1. Lower end of the humerus; 2. the radius; 3. the ulna; 4. the muscle called the round pronator, one of the powerful pronators of the hand. 171. The hand is composed of the carpus, metacarpus, and fingers. 172. The carpus (fig. LXXXII. 4) consists of eight small wedge-shaped bones, placed in a double row, each row containing an equal number, and the whole disposed like stones in an arch (fig. LXXXII. 4). They do in fact form an arch, the convexity of which is upwards, on the dorsal surface (fig. LXXXIII. 4); and the concavity downwards, on the palmar surface (fig. LXXXII. 4). But they differ from the stones of an arch in this, that each bone is joined to its fellow by a distinct moveable joint, each being covered with a smooth articulating cartilage. At the same time all of them are tied together by ligaments of prodigious strength, which cross each other in every direction (fig. LXXVII. 6), so that the several separate joints are consolidated into one great joint. The consequence of this mechanism is that some degree of motion is capable of taking place between the several bones, which, when multiplied together, gives to the two rows of bones such an extent of motion, that when the wrist is bent the arch of the carpus forms a kind of knuckle. By this construction a facility and ease of motion, and a power of accommodation to motion and force, are obtained, such as belong to no arch contrived by human ingenuity. Fig. LXXXII 1. Lower extremity of the radius; 2. lower extremity of the ulna; 3. styloid process of the ulna; 4. bones of the carpus or wrist; 5. metacarpal bones; 6. first phalanges of the fingers; 7. second phalanges of the fingers; 8. third phalanges of the fingers. 173. The metacarpus (fig. LXXXII. 5), the middle portion of the hand, interposed between the wrist and the fingers, is composed of five bones, which are placed parallel to each other (fig. LXXXII. 5). They are convex outwardly, forming the back (fig. LXXXIII. 5), and concave inwardly, forming the hollow of the hand (fig. LXXXII. 5). They are large at each end, to form the joints by which they are connected with the wrist and fingers (figs. LXXXII. and LXXXIII.): they are small in the middle, in order to afford room for the lodgment and arrangement of the 174. Each of the fingers is composed of three separate pieces of bone, called phalanges; the thumb has only two (fig. LXXXII. 6, 7, 8): the phalanges are convex outwardly (fig. LXXXII. 6, 7, 8) for increasing their strength, and flattened inwardly (fig. LXXXIII. 6, 7, 8) for the convenience of grasping. The last bones of the fingers, which are small, terminate at their under ends, in a somewhat rounded and rough surface (fig. LXXXIII. 8), on which rests the vascular, pulpy, and nervous substance, constituting the special organ of touch, placed at the points of the fingers, and guarded on the upper surface by the nail (fig. LXXXII. 8). Fig. LXXXIII. 1. Lower extremity of the radius; 2. lower extremity of the ulna; 3. styloid process of the ulna; 4. bones of the carpus; 5. metacarpal bones; 6. 7. 8. first, second, and third phalanges of the fingers. 175. The round inferior extremity of the metacarpus is admitted into the cavity of the superior extremity of the first phalanx of the five fingers (figs. LXXXII. and LXXXIII.), and their joints are connected by lateral and transverse ligaments of great strength (fig. LXXVII. 9). The situation and direction of the ligaments which unite the several phalanges of the fingers (fig. LXXVII. 9) are precisely the same as those of the articulation of the phalanges with the metacarpus (fig. LXXVII. 7, 8); and the articulation of these bones with one another is such as to admit only of the motions of flexion and extension. 176. The muscles which perform these motions are seated for the most part on the fore-arm. Independently of the supinators and pronators which have been already described (167 et seq.), there are distinct sets of muscles for bending and extending the wrist and the fingers. The flexors arise from the internal, and the extensors from the external, condyle of the humerus (fig. LXIX. 3, 4). The internal condyle is larger and longer than the external (fig. LXIX. 3, 4); for the flexors require a larger point of origin and a longer fulcrum than the extensor muscles; because to the actions of flexion, such as grasping, bending, pulling, more power is necessary than to the action of extension, which consists merely in the unfolding or the opening of the hand previously to the renewal of the grasp. 177. For the same reason, two muscles are provided Fig. LXXXIV. View of the flexor muscles of the fingers. 1. The superficial flexor, divided and turned aside, to show, 2. the deep flexor; 3. sheaths for the tendons of the deep flexor, formed by the splitting of the tendons of the superficial flexor; 4. the anterior annular ligament, divided and turned aside. 118. The muscle that extends the fingers, called the common extensor, is placed on the back part of the fore-arm (fig. LXXXV.), about the middle of which it divides into four portions which terminate in so many tendons (fig. LXXXV. 2). When they reach the back of the metacarpal bones, these tendons become broad and flat, and send tendinous expansions to each other, forming a strong tendinous sheath which surrounds the back of the fingers (fig. LXXXV. 2). These tendinous expansions are inserted into the posterior part of the bones of the four fingers (fig. LXXXV. 2); and their office is powerfully to extend all the joints of all the fingers (fig. LXXXV. 2). 179. On both the palmar and dorsal regions of the wrist are placed ligaments for tying down these tendons, and preventing them from starting from their situation during the action of the muscles (figs. LXXXIV. and LXXXV.). On the palmar region an exceedingly strong ligament passes anteriorly to the concave arch of the carpus (fig. LXXXIV. 4) for the purpose of tying down the tendons of the flexor muscles. On the dorsal surface (fig. LXXXV.), a similar ligament, passing in an oblique direction from the styloid process of the radius to the styloid process of the ulna (fig. LXXXV. 3), performs the same office in tying down the tendons of the extensor muscle. Both these ligaments are called annular. Fig. LXXXV. View of the extensor muscles of the fingers. 1. The common extensor, sending (2 2 2 2) tendons to each finger; 3. the posterior annular ligament. 180. In the palm of the hand are placed additional muscles which assist the flexors of the fingers (fig. LXXXVI. 2), being chiefly useful in enabling the fingers to perform with strength and precision short and quick motions. There are especially four small and rounded muscles (fig. LXXXVI. 2), resembling the earth worm in form and size, and hence called lumbricales; but as their chief use is to assist the fingers in executing short and rapid motions, they have also received the better name of the musculi fidicinales. Fig. LXXXVI. 1. The muscle called the square pronator; 2. muscles seated in the palm of the hand, by which, chiefly, the fingers execute short and rapid motions. 181. The thumb, in consequence of the comparative looseness of its ligaments, is capable of a much greater extent of motion than the fingers, and can be applied to any part of each of the fingers, to different parts of the hand, and in direct opposition to the power exerted by the whole of the fingers and hand, in the act of grasping. The muscles which enable it to perform these varied motions, and which act powerfully in almost every thing we do with the hand, form a mass of flesh at the ball of the thumb (fig. LXXXVII. 1), almost entirely surrounding it. The little finger is also provided with a distinct apparatus of muscles (fig. LXXXVII. 2), which surrounds its root, just as those of the thumb surround its ball, in order to keep it firm in opposition to the power of the thumb in the act of grasping, and in various other motions. Fig. LXXXVII. 1. The mass of muscles forming the ball of the thumb; 2. the mass of muscles forming the ball of the little finger; 3. tendons of one of the flexor muscles of the fingers; 4. sheaths formed by the tendons of the superficial flexor for the reception of the tendons of the deep flexor. 182. The upper extremity is covered by a tendinous expansion or fascia which envelopes the whole arm, encloses its muscles as in a sheath, and affords them, in their strong actions, "that kind of support which workmen feel in binding their arms with thongs." This fascia likewise descends between many of the muscles, forming strong partitions between them, and affording points of origin to many of their fibres, scarcely less fixed than bone itself. 183. From the whole, it appears, that the first joint of the upper extremities, that of the shoulder, is a ball and socket joint, a joint admitting of motion in every direction; that the second joint, that of the elbow, is partly a hinge-joint, admitting of flexion and extension, and partly a rotation joint, admitting of a turning or rotatory motion; and that the joints of the wrist and of the fingers are likewise hinge-joints, admitting at the same time of some degree of lateral motion. When these various motions are combined, the result is that the hand can apply itself to bodies in almost every direction, in any part of the area described by the arm, when all the joints are moved to their utmost extent. There is thus formed an instrument of considerable strength, capable of a surprising variety and complexity of movements, capable of seizing, holding, pulling, pushing and striking with great power, yet at the same time capable of apprehending the minutest objects, and of guiding them with the utmost gentleness, precision, and accuracy, so that there are few conceptions of the designing mind which cannot be executed by the skilful hand. 184. The lower extremities consist of the thigh, leg, and foot. 185. The osseous part of the thigh consists of a single bone, called the femur (fig. XXXIV. 4), the longest, thickest, and strongest bone in the body. It sustains the entire weight of the trunk, and occasionally much heavier loads superimposed upon it. It is constructed in such a manner as to combine strength with lightness. This is effected by rendering the bone what is technically Fig. LXXXVIII. A section of the femur, showing, 1. the compact bony substance; 2. the spongy or cancellated structure; 3. the internal cavity containing the marrow; 4. body; 5, extremities of the bone. 186. The thigh-bone, placed at the under and outer part of the pelvis, has an oblique direction, the under being considerably nearer its fellow than the upper end (fig. XXXIV. 4), in order to afford space for the passages at the bottom of the pelvis, and also to favour the action of walking. The body of the bone, which is of a rounded form (fig. XXXIV. 4), is smooth on its anterior surface (fig. XXXIV. 4), where it is always slightly convex, the convexity being forwards (fig. XXXIV. 4), while its posterior surface is irregular and rough, and forms a sharp prominent line, termed the linea aspera (fig. XXXV. 4), giving attachment to numerous muscles. 187. The superior extremity of the femur terminates in a large ball or head, which forms nearly two-thirds of a sphere (fig. LXXXIX. 4.). It is smooth, covered with cartilage, and received into the socket of the ilium called the acetabulum, which, deep as it is, is still further deepened by the cartilage which borders the brim (fig. LXXXIX. 3). The brim is particularly high in the upper and outer part, because it is in this direction that the reaction of the ground against the descending weight of the trunk tends to dislodge the ball from its socket. 188. Passing obliquely downwards and outwards from the ball, is that part of the femur which is called the neck (fig. LXXXIX. 5). It spreads out archlike between the head and the body of the bone, and is more than an inch in length (fig. LXXXIX. 5). It is thus long in order that the head of the bone may be set deep in its socket, and that its motions may be wide, free, and unembarrassed. Fig. LXXXIX. 1. Lower portion of the ilium; 2. tuberosity of the ischium: 3. socket for the head of the femur, or thigh-bone; 4. head of the femur; 5. neck of the femur; 6. the great process of the femur called the trochanter major; 7. the body of the femur. 189. From the external surface of the femur, nearly in a line with its axis, proceeds the largest and strongest bony process of the body which gives insertion to its most powerful muscles, namely, those that extend the thigh and that turn it upon its axis (fig. LXXXIX. 6). Because, from its oblique direction, it rotates the thigh, this process is called the trochanter, and, from its size, the trochanter major. At the under and inner part of the neck on the posterior surface of the bone, is a similar process, but much smaller, called the trochanter minor (fig. XXXV. 4), into which are inserted the muscles that bend the thigh. 190. The inferior extremity of the femur, much broader and thicker than the superior (fig. XC. 1), is terminated by two eminences, with smooth surfaces, termed condyles (fig. XC. 2), which, articulated with the tibia, and the patella, form the joint of the knee (figs. XC. 2, 4, 5, and XCI. 1, 2, 3). Fig. XC. 1. Lower end of the femur; 2. condyles of the femur; 3. upper end of the tibia; 4. articular surfaces on the head of the tibia on which the thigh-bone plays; 5. the patella, or knee-pan; 6. upper end of the fibula, not entering into the knee-joint. Fig. XCI. Posterior view of the bones forming the knee-joint. 1. Lower end of the femur; 2. upper end of the tibia; 3. articular surfaces on the head of the tibia, on which the thigh-bone plays; 4. upper end of the fibula, not entering into the knee joint. 191. The bones of the leg, two in number, consist of the tibia (fig. XC. 3) and fibula (fig. XC. 6). The tibia, next to the femur, the longest bone in the body, is situated at the inner side of the leg (fig. XC. 3). Its superior extremity is bulky and thick (fig. XC. 3). The top of it forms two smooth and slightly concave surfaces, adapted to the convex surfaces of the condyles of the femur (fig. XC. 4, 2). On its outer side there is a smooth surface, to which the head of the fibula is attached (fig. XC. 6). Its lower extremity, which is small, forms a concavity adapted to the convexity of the bone of the tarsus, called the astragalus, with which it is articulated (fig. XCII. 4.) Its inner part is produced so as to form the inner ankle (figs. XCII. 2, and XCIII. 3): its outer side is excavated into a semilunar cavity, for receiving the under end of the fibula, which forms the outer ankle (figs. XCII. 3, and XCIII. 4). 192. The fibula, in proportion to its length the most slender bone of the body, is situated at the outer side of the tibia (fig. XC. 6). Its upper end formed into a head, with a flat surface on its inner side (figs. XC. 6, and XCI. 4), is firmly united to the tibia (fig. XC. 4). Its lower end forms the outer ankle, which is lower and farther back than the inner (fig. XCII. 3, 2). Fig. XCII. Anterior view of the bones forming the ankle-joint. 1. Lower end of the tibia; 2. production of the tibia, forming the inner ankle; 3. lower end of the fibula, forming the outer ankle; 4. upper part of the astragalus: these three bones form the ankle-joint; 5 5 5, other bones of the tarsus; 6 6 6 6 6 metatarsal bones. Fig. XCIII. Posterior view of the bones forming the ankle-joint. 1. Lower end of the tibia; 2. lower end of the fibula; 3. internal malleolus or ankle; 4. external malleolus or ankle; 5. one of the tarsal bones, called the astragalus, with which the tibia and fibula are articulated; 6. the os calcis or heel. 193. The patella, or knee-pan (fig. XC. 5), is a light but strong bone, of the figure of the heart as painted on playing-cards, placed at the fore part of the joint of the knee, and attached by a strong ligament to the tibia, the motions of which it follows (fig. XC. 5). It is lodged, when the knee is extended, in a cavity formed for it in the femur (fig. XC.); when bent, in a cavity formed for it at the fore part of the knee (fig. XC. 5). 194. The foot consists of the tarsus, metatarsus, and toes. 195. The tarsus, or instep, is composed of seven strong, irregular-shaped bones, disposed like those of the carpus, in a double row (fig. XCII. 4, 5). The arrangement of the tarsal bones is such as to form an arch, the convexity of which above, constitutes the upper surface of the instep (fig. XCII. 4, 5): in the concavity below are lodged the muscles, vessels, and nerves that belong to the sole. 196. The metatarsus consists of five bones, which are placed parallel to each other (fig. XCII. 6), and which extend between the tarsus and the proper bones of the toes (fig. XCII. 6). Their extremities, especially next the tarsus, are large, in order that they may form secure articulations with the tarsal bones (fig. XCII. 6). Their bodies are arched upwards (fig. XCII. 6), slightly concave below, and terminate forwards in small, neat, round heads, which receive the first bones of the toes, and with which they form joints, admitting of a much greater degree of rotation than is ever actually exercised, in consequence of the practice of wearing shoes. The natural, free, wide-spreading form of the toes, and the consequent security with which they grasp the ground, is greatly impaired by this custom. Taken together, the bones of the metatarsus form a second arch corresponding to that of the tarsus (fig. XCVIII. 2). 197. Each toe consists of three distinct bones, 198. Besides the bones already described, there are other small bones, of the size and figure of flattened peas, found in certain parts of the extremities, never in the trunk, called sesamoid, from their resemblance to the seed of the sesamum. They belong rather to the tendons of the muscles than to the bones of the skeleton. They are embedded within the substance of tendons, are found especially at the roots of the thumb and of the great toe, and are always placed in the direction of flexion. Their office, like that of the patella, which is, in truth, a bone of this class, is to increase the power of the flexor muscles by altering the line of their direction, that is, by removing them farther from the axis of the bone on which they are intended to act. 199. The ligaments which connect the bones of the lower extremities are the firmest and strongest in the body. Of these, the fibrous capsule of the hip-joint (fig. XCIV. 1), which secures the head of the femur in the cavity of the acetabulum (fig. XCIV.), is the thickest and strongest. It completely surrounds the joint (fig. XCIV. 1). It arises from the whole circumference of the acetabulum, Fig. XCIV. 1. The fibrous capsule of the hip-joint, laid open and turned aside to show, 2. the round ligament in its natural position. Fig. XCV. A view of the head of the femur drawn out of its socket, and suspended by the round ligament, to show more clearly the action of the ligament in retaining the head of the femur in its socket. 200. Numerous and complicated ligaments connect the bones that form the knee-joint (fig. XCVI.), and the strength of these powerful bands Fig. XCVI. General view of the ligaments of the knee-joint. 1. Lower end of the femur; 2. upper end of the tibia; 3. upper end of the fibula; 4. the patella; 5. united tendons of the extensor muscles; 6. ligaments of the patella; 7. the capsular investment of the knee; 8. the internal lateral ligament; 9. the external lateral ligaments; 10. the posterior ligament; 11. the ligament connecting the tibia and fibula; 12. a portion of the interosseous ligament. 201. Strong ligaments maintain in their proper position the bones that form the ankle-joint (fig. Fig. XCVII. General view of the posterior ligaments of the ankle-joint. 1. Lower end of the tibia; 2. lower end of the fibula; 3. astragalus; 4. os calcis; 5. ligament between the tibia and fibula; 6. ligament passing from the fibula to the astragalus; 7. ligament passing from the fibula to the os calcis; 8. ligament passing from the tibia to the astragalus. Fig. XCVIII. General view of the ligaments of the sole of the foot. 1. Ligaments connecting the bones of the tarsus; 2. ligaments connecting the bones of the toes. 202. The joint of the hip, like that of the shoulder, is capable of flexion, extension, and rotation; but its rotatory motions are to a much less extent, on account of the greater depth of the acetabulum and the stronger and shorter fibrous capsule. When the femur is flexed, the thigh is bent upon the pelvis, and its inferior extremity is 203. The apparatus of muscles that produces these varied motions is seated partly on the trunk and partly on the pelvis. Thus, the powerful muscle that flexes the thigh, or that carries it forwards, termed the psoas (fig. XCIX. 1), arises from the last vertebra of the back, and successively from each vertebra of the loins (fig. XCIX. 1), and is inserted into the lesser trochanter of the femur (fig. XCIX. 3). Its action is assisted first by a large and strong muscle named the iliacus (fig. XCIX. 2), which occupies the whole concavity of the ilium (fig. XCIX. 2), and which, like the psoas, is inserted into the lesser trochanter of the femur (fig. XCIX. 3). Fig. XCIX. View of the muscles that bend the thigh. 1. The muscle called psoas; 2. the muscle called iliacus; 3. tendons of these muscles, going to be inserted into the trochanter minor of the femur. 204. The muscles that extend the thigh, or that carry it backwards, named the glutÆi, the most powerful muscles of the body, are placed in successive layers, one upon the other, on the back part of the ilium (fig. C. 1, 2, 3), and are inserted into the linea aspera of the femur. They constitute the mass of flesh which forms the hip, and their powerful action in drawing the thigh backwards is assisted by several other muscles (fig. C. 4, 5, 6). Their action is never perfectly simple and direct; for those which move the thigh forwards sometimes carry it inwards, and sometimes outwards; and in like manner, those which move it backwards, at one time carry it inwards and at Fig. C. View of the muscles that extend the thigh. 1. The muscle called glutÆus maximus, removed from its origin, 2, 2, to show the muscles which lie beneath it; 2. cut edge showing the origin of the same muscle; 3. the muscle called glutÆus medius; 4, 5, 6. smaller muscles, assisting the action of the glutÆi. 205. The knee is a hinge-joint, admitting only of flexion and extension, and is therefore provided only with two sets of muscles, one for bending and the other for extending the leg. The flexors of 206. Four large muscles, blended together in such a manner as to form one muscle of prodigious size, termed the quadriceps cruris (fig. CI. 7), occupying nearly all the forepart and the sides, and a considerable portion of the back part of the thigh, constitute the great flexor of the thigh. This enormous mass of muscle arises partly from the ischium, and partly from the upper part of the femur (fig. CI. 7), and is all inserted into the patella (fig. CI. 8), which constitutes a pulley for the purpose of assisting the action of these powerful muscles. 207. The muscles which bend the toes and extend the foot, termed the gastrocnemii (fig. CII. 1, 2), are placed on the back part of the leg, and form the mass of muscle which constitutes the calf Fig. CII. View of the muscles which bend the toes, and which, by lifting the heel, extend the foot. 1. The muscle called gastrocnemius externus, which, uniting with 2. the gastrocnemius internus, forms 3. the tendo achillis, which is inserted into 4. the heel. Fig. CIII. View of the muscles which extend the toes and bend the foot. 1. The common extensor; 2. the tendons of the same muscle inserted into the toes; 3. the anterior annular ligament of the foot. Fig. CIV. View of the muscles in the sole of the foot. 1 The muscle which draws the great toe from the other toes; 2. the muscle which draws the little toe from the other toes; 3. the muscle called the short flexor of the toes, which assists in bending the four smaller toes. 208. The muscles which extend the toes and bend the foot are seated on the fore part of the leg 209. Such are the moving powers which put in action the complicated mechanism provided for the function of locomotion. And these powers are adequate to their office; but they are what may be termed expensive powers; agents requiring a high degree, of organization and the utmost resources of the economy to support and maintain them. Hence in the construction of the framework of the machine which they have to move, whatever mechanical contrivance may economize their labour, is adopted. The construction, form, and disposition of the several parts of that framework have all reference to two objects: first, the combination of strength with lightness; and secondly, security to tender organs, with the power of executing rapid, energetic, and, sometimes, violent motions. The combination is effected and the object attained in a mode complicated in the detail, simple in the design, and perfect in the result. The weight of the body transmitted from the arch of the pelvis to a second arch, formed by 210. It is necessary, in order to complete this general view of the structure of the human body, and of the combination and arrangement of its various parts, to denote the several regions into which, for the purpose of describing with accuracy the situation and relation of its more important organs, the body is divided. It is not needful to the present purpose to describe the regions of the head, because its internal cavity contains only one organ, the brain, and its external divisions do not differ materially from those which are common Fig. CV. Anterior view of the regions of the body. 1. Region of the neck; 2. region of the chest or thorax. Abdominal regions: 3. epigastric; 4. umbilical; 5. hypogastric region. Regions of the upper extremities. 6. shoulder; 7. arm; 8. elbow; 9. fore-arm; 10. wrist; 11. ball of thumb; 12. the axilla or armpit. Regions of the lower extremities: 13. thigh; 14. knee; 15. leg; 16. ankle; 17. instep and foot. Fig. CVI. Posterior view of the regions of the body: 18. region to the scapula; 19. of the back; 20. of the loins; 21. of the hips; 22. of the ham; 23. of the calf of the leg; 24. of the heel and foot. Fig. CVII. Lateral view of the regions of the body: 25. arch of the foot. Fig. CVIII. Anterior view of the situation of the more important internal organs: 1. lungs, right and left; 2. heart; 3. line representing the edge of the diaphragm; 4. liver; 5. stomach; 6. small intestines; 7. colon; 8. urinary bladder. Fig. CIX. Posterior view of the situation of the more important internal organs: 9. kidnies, right and left; 10. the course of the spinal cord. Fig. CX. Lateral view of the situation of the more important internal organs. |
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