Jabez Hogg

The most complicated condition in which matter exists is where, under the influence of life, it forms bodies with a structure of tubes and cavities in which fluids are incessantly in motion, and producing continuous changes. These have been rightly designated “organised bodies,” because of the various organs they contain. The two principal classes into which organised bodies have been divided are recognised as vegetable and animal. It was Bichat who taught that our animal life is double, while our organic life is single. In organic life, to stop is to die; and the life we have in common with vegetables never sleeps, and if the circulation of the fluids within the animal body ceases for a few seconds, it ceases for ever. In the vertebrate body, however, the combination of organs attains to the highest development, in striking contrast with that of the class we have previously considered, the Invertebrata, the animal kingdom being divided into Vertebrates and Invertebrates.

The Vertebrata are distinguished from all other animals by the circumstance that a transverse and a vertical section of the body exhibits two cavities completely separated from one another by a partition. A still more characteristic feature separates the one from the other; it is the specialisation of the chief nervous centres, and their peculiar relation to the other systems of the body.

The dorsal cavity of the body contains the cerebro-spinal nervous system, the ventral, the alimentary canal, the heart, and usually a double chain of ganglia; these pass under the name of the sympathetic system. It is very probable that this sympathetic nervous system represents, wholly or partially, the principal nervous system of the Annulosa and Mollusca. In any case, the central parts of the cerebro-spinal nervous system—i.e., the brain and the spinal cord—would appear to be unrepresented among invertebrate animals. Likewise, in the partition between the cerebro-spinal and visceral tubes, certain structures which are not represented in Invertebrates are contained. During the embryonic condition of all Vertebrates, the centre of the partition is occupied by an elongated cellular cylindrical mass, the notochord, or chorda dorsalis. This structure persists throughout the life in some Vertebrata, but in most it is more or less completely replaced by a jointed, partly fibrous, cartilaginous, and bony vertical column. All vertebrate animals have a complete vascular system. In the thorax and abdomen, in place of a single perivisceral cavity, in communication with the vascular system, and serving as a blood-sinus, there are one or more serous sacs. These invest the principal viscera, and may or may not communicate with the exterior, recalling in the latter case the atrial cavities of the Mollusca. In all Vertebrata, except Amphioxus, there is a single valvular heart, and all possess a hepatic portal system, the blood of the alimentary canal never being wholly returned directly to the heart by the ordinary veins, but being more or less completely collected into a trunk (the portal vein), which ramifies through and supplies the liver.

With reference to one other point of importance, the development of the ova of Vertebrates, these have the same primary composition as those of other animals, consisting of a germinal vesicle containing one or more germinal nuclei, and included within a vitellus. But as this forms a part of general anatomy, and as my object is simply the investigation of the fundamental and microscopical structure of animal organisms, I shall not further pursue the morphological part of the subject, especially as so many excellent text-books are within reach of the student who desires to fully acquaint himself with precise information.

Notwithstanding, then, the apparent diversity in the structure of the vertebrate and the invertebrate and the various tissues of which animals and vegetables are constituted, microscopical research has satisfactorily demonstrated that all textures have their origin in cells; in fact, when the formative process is complete, the animal cell is seen to consist of the same parts and almost the same chemical constituents as the typical cell of the plant—namely, a definite cell-wall enclosing cell contents, of which the nature may be diverse, but the cell nucleus is precisely the same and is the actual seat and origin of all formative activity. The cell and nucleus grow by assimilation or intersusception, that is, by inflowing of nutrition among all parts, the new replacing the old, yet maintaining its original structure and composition. That which was once thought special to animals is now found to be common to both plants and animals: they are found to be alike fundamentally in internal structure, and in the discharge of the mysterious processes of reproduction and of nutrition, although the latter forms a convenient line of separation. Life in plants goes on indefinitely; cuttings may be taken without injury to their vigour and duration of life. The same may be said of some of the lower forms of invertebrate life; for example, the hydra, the anemone, and some other well-known animals, may be cut up, divided into several parts, each one of which will form a new animal, provided a nucleus be included in the section. Nevertheless, the organisation of the amoeba and the hydra is as complete for its purpose as that of man for his, and the evidence of continuity forbids the drawing of hard and fast lines, as was formerly done between the two kingdoms, the animal and vegetable. The amount of similarity or agreement in the organisation of animals is various. Animals indeed differ from each other in slight points only, for the discovery of which the microscope must be brought into requisition. Living matter in its earliest stage and simplest form appears to the naked eye as a homogeneous structure, but when placed under the highest powers of the microscope, it is seen not to be so.

But perhaps the most marked feature of the age has been the increasing attention given to the study of the lower forms of life, using their simpler structures and more diffuse phenomena to elucidate the more general properties of living matter. To understand life we must understand protoplasm. Of this there can be no doubt, as we have seen in a previous chapter that a whole family, the Monera, consists of this simple living, microscopic, jelly-like substance, which has not even begun to be differentiated, as in the amoeba, which has as yet no special organs, and every speck becomes a mouth or a stomach, and which can be turned inside out and shoot out tongues of jelly to move and feel with. “Reproduction is the faculty most characteristic of life, and sharply distinguishes the organic from the inorganic.” It is, then, the corpuscles of protoplasm, called cells (cellulÆ), which have so much interest for the physiologist, and these, like the cytods, may form independent organisms, which are then termed unicellular. Again, cells form other cells, and a multicellular organism results, and goes on increasing in geometrical progression. In the Vertebrata the cell retains its characteristic spheroidal shape, as seen in Fig. 423, and undergoes division by virtue of its living protoplasmic mass.

Fig. 423.

1. Newly formed cell structure; 2. Division of the nucleus; 3. It changes its situation in the cell; 4. Subdivides and breaks up; 5. Cell-walls increase in thickness; 6. Branch out into stellate cells; 7. Two cells coalesce; 8 and 9. Become multicellular.

Epithelial Cells.—All free surfaces of the human body, both internal and external, are to a very considerable extent covered by epithelium cells. These cells are everywhere the same, but with modifications in shape and arrangement. Epithelial cells are nucleated and always joined by their surfaces or edges, without, on the external surfaces, the intervention of connective tissue.

There are four essential varieties:—1. Tesselated; 2. Columnar; 3. Spheroidal; 4. Ciliated; in all of which the nucleus remains remarkably uniform in its characters, is either round or oval, and flattened out, measuring ¹/₆₀₀₀th to ¹/₄₀₀₀th of an inch in diameter. They are insoluble in acetic acid, colourless, or slightly tinted by the structure with which they are in contact, and usually contain one or more nucleoli with a few minute irregular granules, as represented in Fig. 424.

The simplest and most commonly distributed variety is the tesselated, known also as the scaly, squamous, pavement, and flattened epithelium, always arranged in single layers, lining serous cavities, many parts of the mucous membrane, and the interior of ducts and blood vessels. Upon the external surface of the body it occurs in superimposed layers, forming the “stratified epidermis.” To obtain specimens of lamellar epithelium it is only necessary to collect a little saliva, or pass a glass slide over the lining membrane of the cheek, cover it with a thin cover glass, and examine it with a ¼-inch objective. Pavement epithelium is the elementary structure of hair, nails, and horn.

PLATE XIX.

ANIMAL TISSUES.

Columnar epithelium exists upon the mucous membrane of the stomach, on the villi of the intestines, and in the several canals. It occupies either a vertical or horizontal position, and may be detached in rows, as shown in Plate XIX., No. 2, a section taken from the intestine of a rabbit. This variety, when more highly magnified, as in Fig. 424, is seen to consist of club-shaped nucleated cells, the thicker end being turned towards the surface. The protoplasm of the cell is granular, and the presence of minute vacuoles and fatty globules occupy a great part of the space. The nucleus is now seen to contain a fine network. At times the outer end of the cell is distended, as in Fig. 3. This form of columnar epithelium (known as the “goblet” cell) presents a close and remarkable resemblance to the cilio-flagellate “collared” infusorial monad in its extended “wine-glass” form.

Fig. 424.

No. 1. Pavement epithelium, taken from an internal membrane; 2. Columnar epithelium, from the intestine of a rabbit, showing central fat globules, and at str a fine ciliated border; 3. A so-called “goblet”-cell.

Spheroidal epithelium is confined to the closed cavities of the body, and in the internal structure of the ducts of secreting glands. The cells are, for the most part, circular, although some are flattened out at the sides in which they are in contact with each other (Plate XIX., No. 1a). Specimens of this form may be taken from the internal surface of one of the lower animals with a scalpel. The collected matter must be placed in a drop of distilled water and examined with a high power.

Ciliated epithelium is characterised by the presence of those fine hair-like filaments (cilia) attached to the free surface of the cell. During life, and for some time after death, the cilia are seen to retain their constant waving motion. The cilia all move in one direction and rhythmically, thus giving rise to the appearance of a succession of undulations. Ciliated epithelium is found lining the mucous membrane of the air passages and nasal ducts, and wherever it is necessary to urge on a secretion by mechanical means, ciliated epithelium exists. Specimens for examination are easily obtained from the oyster, and with care will show the characteristic motion. A portion of a gill separated from the mollusc will live on for a considerable time if kept in a little of its natural secretion. The parameciÆ, rotifera, and all the ciliata, are furnished with cilia as a means of locomotion and obtaining sustenance. By snipping off a small piece from the gills of the mussel, always accessible to the microscopist, and covering it over with thin glass to prevent evaporation of the animal juices, its cilia will continue to work for hours.

Lymph and Blood, Fig. 425 B, a a.—There are other cells in the animal body which possess a certain amount of resemblance to those confined to the more superficial structures—i.e., the lymph, chyle, and blood. These fluids present in one respect a physical uniformity of composition, and a resemblance in the size of their characteristic corpuscles. Chyle contains besides the corpuscles of lymph, a quantity of minute granules which imparts a white colour to the fluid. Intermixed are oil globules, free nuclei, and sometimes a few red blood discs. Chyle may be had for microscopic examination by squeezing a little juice from the lymphatic gland of a sheep just slaughtered.

Fig. 425.—Human Blood Corpuscles and Crystals.

A. a a. Red blood corpuscles lying flat on the warm stage; b b. in profile; c c. arranged in rouleaux; d. crenated; e. rendered spherical by water; I. leucocytes and white amoeboid corpuscles; B. Blood discs of fowl, red and white, others seen in convexity and with a nucleus. Blood Crystals.—C. HÆmatin from human blood; D. HÆmatoidin; E. HÆmin; F. Tetrahedral; G. Pentagonal; H. Octahedral crystals from blood of mouse.

Blood Corpuscles or cells vary considerably in mammals, birds, reptiles, and fishes. Fig. 102 (page 143) is a microphotograph of a drop of blood magnified 3,500 times; and Fig. 425, A, shows both red and white discs drawn to scale, magnified 1,200 diameters. The red corpuscles of human blood are distinguished by their clearly defined outlines and dark centres. Each disc is biconcave in form, and hence the whole surface cannot be focussed at the same time. When the circumference is well illuminated the centre is dark, but by bringing the objective nearer to the object, the concavity of the disc is brought into focus. It generally happens that blood corpuscles, on being first drawn, run together, and present the appearance of rolls of coins; or they may be scattered about over the field. There is a considerable difference in the form of the discs; they are circular in all mammals, except the camel, dromedary, and llama, these being oval. In profile blood corpuscles are biconcave, their investing membrane is homogeneous and elastic, and will readily move along the smallest capillary vessels. There is no trace of a nucleus in the blood-discs of the adult Mammalia, while in size they bear no proportion to the bulk of the animal in whose blood-vessels they circulate. The corpuscles of Mammalia in general are like those of man in form and size, being either a little larger or smaller. The most marked exception is the blood of the musk-deer, in which the corpuscles are of extreme smallness, about the ¹/₁₂₀₀₀th of an inch in diameter. In the elephant they are large, about ¹/₂₇₀₀th of an inch in diameter. The goat, among common animals, has very small corpuscles, but they are, withal, twice as large as those of the musk-deer. In the Menobranchus lateralis they are of a much larger size than in any animal, being the ¹/₃₅₀th of an inch; in the proteus, the ¹/₄₀₀th of an inch in the longest diameter; in the salamander, or water-newt, ¹/₆₀₀th; in the frog, ¹/₉₀₀th; lizards, ¹/₁₄₀₀th; in birds, ¹/₁₇₀₀th; and in man, ¹/₃₂₀₀th of an inch. Of fishes, the cartilaginous have the largest corpuscles; in gold-fish, they are about the ¹/₁₇₀₀th of an inch in their longest diameter.

The large size of the blood discs in reptiles, especially in the Batrachia, has been of great service to physiologists by enabling them to ascertain many particulars regarding structure which could not have been otherwise determined with certainty. The value of the spectroscope in the chemical examination of the blood has been already referred to. See page 252.

White corpuscles or leucocytes (Fig. 425, I) differ materially from the red. They are large, spheroidal, finely granular masses of about ¹/₂₈₀₀th of an inch in diameter. In a cubic millimÈtre of human blood there are about 10,000 white corpuscles. They have a lower specific gravity than the red, have no cell-wall, and their substance mainly consists of protoplasm. The internal granular appearance is now believed to be due to a fine intercellular network having small dots at the intersections of the web. In the meshes of the net a hyaline substance is interspersed. They possess one or more nuclei; these are seen on the application of a few drops of acetic acid. When examined in a perfectly fresh state, especially if the glass slide be placed on the warm stage of the microscope, they exhibit a spontaneous change of shape, amoeba-like, such movements being accordingly termed amoeboid. The movements referred to consist in the protrusion of processes of protoplasm which are retracted and other processes protruded as represented (Fig. 425, I). Both in human blood and in newts there are colourless corpuscles which contain coarser granules than others; these are called granular corpuscles. Some are shown near the amoeboid bodies. The white corpuscles are readily found in various tissues of the body, as in the lymphatic glands. In inflammatory diseases these leucocytes pass through the walls of the capillaries into the tissues, and form morbid products, pus-cells.

Sections of blood discs are made by dipping a fine needle in a drop of blood as it exudes from a prick of the finger and drawing thin lines across the glass slip, allowing time to dry, and then cutting the lines across in all directions with a razor. The loosened portions should be removed with a camel’s-hair brush.

In birds, the blood discs are oval in shape and possess a nucleus, shown in Fig. 425 B, in the blood of the fowl; this is rendered more apparent on adding a drop of acetic acid. The blood of fishes is also oval and nucleated, rather more pointed than that of birds. In reptiles generally the red blood discs are large, oval, nucleated bodies, the white corpuscles still preserving their invariable circular form and granular appearance. In the salamander and proteus the discs attain to their greatest size. In the former they measure ¹/₇₀₀th of an inch, and in the latter ¹/₄₀₀th.

Blood Crystals.—In addition to the elements described, the blood contains various crystalline forms, represented in Fig. 425, C to H. In connection with the micro-spectroscope (p. 253), the spectra of certain blood crystals are given; although varying in different animals, sufficient uniformity prevails as to render them characteristic. The crystals are formed when a little blood is mixed with water on the slide, allowing a short time for crystallisation. Near the edge of the cover-glass, where crystals begin to form, they are more distinct, but a high power is required for their examination. In human blood the crystals are prismatic; in that of the guinea-pig, tetrahedral; in the blood of the mouse, octahedral. Other forms may be obtained by the aid of chemical reagents.

In human blood there are at least three distinct forms of crystals: HÆmatin is formed in normal blood, is made visible on the addition of a little water to blood, or by agitation with ether, so as to dissolve the cell-wall of the blood corpuscles, and allow the contents to escape. A drop of blood will furnish crystals large enough to be seen with a moderate power. HÆmatoidin crystals are abnormal products, found in connection with certain diseased conditions. These crystals are seen as represented at D. HÆmin crystals must be regarded as artificial chemical products, the result of treating blood with glacial acetic acid; the acicular crystals at E, reddish-brown in colour, are artificially produced.

Fig. 426.

1. White fibrous or non-elastic tissue; 2. Yellow fibrous elastic tissue.

Basement Membrane—Connective Tissue System.—Connective or areolar tissue is present almost throughout the whole of the human body, and serves to connect the various organs with one another, as well as to bind together the several parts. The muscles are surrounded by a connective tissue sheath; this penetrates into their substance, and binds together fasciculi and fibres. The same tissue is present in the skin and the mucous membranes; it also forms a sheath for the arteries, veins, and nerves. It is plentifully supplied by blood-vessels, and nerves pass through its substance. Microscopically, four different elements can be clearly made out:—1. Connective tissue cells or corpuscles; 2. White fibrous tissue; 3. Yellow fibrous tissue; 4. Ground substance.

On examining the connective tissue cells of young animals, various cells will be seen with fine granular contents, together with nuclei, lying in spaces in the ground substance, some branched, others flattened or rounded. Even tissues supposed to be homogeneous in structure, are on staining seen to have connective tissue cells, such as those represented in a section of the cornea of the eye (see p. 31). In this case the connective tissue cells are termed corneal corpuscles; the branched cells, it will be noticed, are united by branches.

The cells in the fibrous tissue of tendons are square or oblong, and form continuous rows. White fibrous tissue is distributed throughout the animal body, but in a variety of forms; it is found in the skin and other membranes, and in all parts where strength and flexibility are necessary. The structure of white and yellow fibrous tissues is shown in Figs. 426 and 427.

White fibrous tissue presents silver-lustre bundles, running for the most part in parallel directions through and over the muscles and tendons. For examination under the microscope, obtain a fragment of fresh meat cut in the longitudinal direction; place it in water, and tease it out with needles as directed in a former chapter. The smallest fragment will suffice for examination under a quarter or one-sixth inch objective. These filaments are exceedingly minute, measuring ¹/₃₀₀₀th to ¹/₂₅₀₀th of an inch in diameter, and do not interlace through the bundles, although they intersect each other occasionally. Transverse sections may be made by drying a piece of tendon until it becomes sufficiently firm to cut with a razor or microtome, and mounted as a permanent specimen. From the cut ends of the fibres small dark points will be seen, especially in the denser structure of the tendons; these are termed “connective tissue corpuscles.”

Yellow elastic fibrous tissue is remarkable in contradistinction to the white for its elasticity and capability of extension. It is found on the coats of blood-vessels, between the vertebral arches, and in quadrupeds it forms a strong elastic band, extending from the occiput, throughout the spines of the vertebra, and enabling the animal to support the head in the pendent position, without muscular exertion. These fibres can only be separated from each other with difficulty, and their elasticity is shown by a tendency to curl up. These yellow fibres are somewhat coarser than the white, and they remain unaffected by acetic acid of the ordinary strength. Elastic tissue is a constituent of the skin, mucous, and serous membranes, and of the areolar or cellular tissue.

In order to microscopically examine this structure, take a small portion of the strong ligament of the neck of the ox, place it as before in water, and tease it out with needles; place a fragment on a glass slip, cover with a thin cover-glass, and submit it to a high magnifying power. Transverse sections made as directed in the case of white tissue will be seen to be hexagonal in form.

Adipose Tissue.—Fat is found in many situations in the animal body, and on examination is seen to consist entirely of vesicles, distributed through a delicate membrane of connective tissue, shown in Plate XIX., Nos. 4 and 5. On pressure, the circular or oval form of the cells becomes polyhedral; occasionally the fatty acids in the interior of the vesicles crystallise, and give rise to a star-like appearance. For the examination of adipose tissue, take a portion of the mesentery of any small animal—a mouse, or rat.

Retiform Tissue.—Adenoid, or retiform tissue, consists of a delicate network of connective tissue corpuscles, joining their branches together. This forms the stroma or framework of lymphoid tissue. It is found in connection with all the lymphatic glands, spleen, &c. Plate XIX., No. 3, a b, shows small sections of a lymphatic, together with capillary vessels.

Muscular Fibre.—There are two varieties of muscular fibre in the body—i.e., striated, and non-striated. The striated is formed in muscles attached to bony structures, as those of the arm and leg, and in some of the soft structures, as the tongue, palate, oesophagus, in short, all muscles under the control of the will. Striped muscle is of a dull red colour and marked with peculiar longitudinal furrows on its surface. Voluntary muscle consists of:—1, a connective tissue sheath; 2, fasciculi; 3, fibres and sarcolemma; 4, discs, fibrilla and sarcous elements. These are shown in connection with other tissues in Plate XIX., Nos. 11 and 12, and also in Fig. 428 (1, 2, 3).

In Plate XIX., Fig. 11, the muscular fibre taken from the tongue of a lamb shows the continuity of the upper portion with the connective tissue membrane. In Fig. 12, a branching-out bundle of muscular fibre, taken from the upper lip of the rat, is seen to end in stellate connective cells. The delicate homogeneous sheath that binds the fibres together is termed sarcolemma. This is readily seen in prepared muscle of the frog and water-beetle, less plainly in man. Each muscle is provided with a sheath of connective tissue; this surrounds it, binds the fasciculi together, and supports the blood-vessels; it is called the perimysium, and sends fine prolongations in between the fibres, termed endomysium. The intervals seen on high amplification between the dark striÆ are called Kruse’s membrane. On breaking up the striated structure it is resolvable into fibrillÆ and furthermore into discs.

Among mammalia the pig furnishes the best examples of muscle fibrillÆ; among insects the water-beetle and the thorax of the housefly. A power of 600 or 800 diameters is required to separate the fibrillÆ. Blood-vessels are well supplied with striated muscle, but none of their minuter branches penetrate the sarcolemma. The involuntary or non-striated variety of muscular fibre exists in all parts of the body where movements occur independently of the will, also in the ciliary muscle and the iris of the eye, as well as in the middle coats of the arteries. Non-striated fibres are pale in colour, prismatic in shape, and easily flattened by pressure. In size, they vary from ¹/₇₀₀₀th to ¹/₃₅₀₀th of an inch in diameter, and are marked at short intervals by oblong corpuscles.

The Integument or Skin consists of epidermis or cuticle, dermis, corium or cutis vera, sweat-glands, nails, hairs, sebaceous glands, and numerous nerves and vessels. The epidermis forms a protective covering over the whole surface of the body, and is moulded on to the surface of the corium beneath, covering the ridges, depressions and papillÆ. It is made up of three principal layers: the horny layer or stratium corneum, the most superficial, this consists of layers of flattened cells, which are without a nucleus; the stratum lucidum, composed of layers of nucleated cells, more or less indistinct in section; the rete mucosum or malpighian layer; is composed in its upper part of layers of “prickle cells” and its inferior of a single stratum of columnar cells. Pigment is principally found in the lowest layer, Fig. 429.

The gradations of colour in the skin are due to the granular contents of the pigment cells. This is seen on steeping sections cut from the skin of a negro in chlorine; the colour is discharged. In Plate XIX., No. 13, the pigment cells of the choroid coat of eye are shown. Here the pigment is darker in colour, and its function is the absorption of light and the prevention of disturbing effects occasioned by circles of dispersion.

The Dermis, or true skin, consists of an interlacing network of connective tissue, yellow elastic tissue corpuscles, vessels, and nerves. There are also small muscular fibres in connection with the hair follicles, and beneath the subcutaneous tissues contain an abundant supply of fat adipose tissue. Numerous ridges are seen on the surface, especially on the palm of the hand and sole of the foot, caused by rows of little elevations of the cutis vera, termed papillÆ. These are more or less conical, and contain a capillary loop, nerve, and touch corpuscle, which serve to increase the sensitiveness of the part, lodging a touch corpuscle in a favourable position for receiving sensations of touch, Fig. 430.

Sweat glands are situated in the subcutaneous tissue, and consist of fine tubes, which form the duct (seen in the section, Fig. 430); these are continuous with a blind extremity, coiled up into a ball one-sixtieth of an inch in diameter, and surrounded by a plexus of capillaries to form the gland (Fig. 431, No. 2). Between the layer of columnar cells and the limiting membrane is a layer of non-striated muscle, and beneath the rite mucosum there are several layers of polyhedral cells, and an external and internal limiting membrane; the epithelium of the duct is at its mouth continuous with the epithelium of the epidermis.

Nails consist of a root and body, the lunular of which is the whitish portion of the body near the root, where the skin beneath is less vascular than any other portion of the finger. The nail closely resembles the epidermis, and consists of hard and thin layers of cells on the surface, and round, moist cells beneath. Posteriorly the nail fits into a groove which lodges its root. The part to which the nail is attached is known as the nail-bed. The stratified appearance produced by the coalescence of the cells, and their lying over each other, is shown in Plate VII., No. 149, the toe of the mouse; while the special arrangement of tissue is better seen under polarised light (Plate VIII., No. 174).

Hairs consist of a shaft and root. The shaft is cylindrical, and covered with a layer of imbricated scales, arranged with their edges upwards. The substance of the hair consists of fibres, or elongated fusiform cells, in which nuclei are seen. There are present in some hairs (Fig. 432) small air spaces or lacunÆ. In the coarser hair of the body there is a pith (medulla), occupied by small angular cells and fat granules.

The root of the hair is seen to dilate that it may fit more firmly into the skin hair-follicle. The latter consists of two coats, an outer and an inner, continuous with the epidermis, and this is called the root sheath. The outer portion consists of three layers, formed of connective tissue, blood-vessels, and nerves. The inner, or epidermic, coat comes away when the hair is pulled out, and hence is called the root sheath. This again is made up of two layers, the outer of which corresponds with the horny layer, and is composed of flattened cells. The bulbous root of the hair is connected with the papilla. In the cat the tactile nasal hairs are very large. Small bundles of involuntary muscular fibres connect the corium with the root, so that in contracting they elevate or expand the hair.

The hair of the lower animals presents a diversity of structure, especially on the outer surface, and with reference to the arrangement of the scales. The hair of the Indian bat, for instance, consists of a shaft invested with erectile scales, placed at regular intervals; these stand out from the shaft, as in Fig. 433, No. 1. This form of scale varies considerably in the different species of these animals, and a portion of hair near the root is nearly divested of scales. Many of the scales are not unlike those of certain of the insect tribe, seen in that of Dermestes, No. 3, while the hair of the mouse has a series of transverse imbricated scales arranged as tiles on a house, due to accumulated pigment. Hairs taken from various animals form interesting objects of study for the microscope, as already noticed. Other hairs are shown in Fig. 434. No. 1 is a transverse section of a hair from the ant-eater; the central part consists of air-cells, the outer of a granular pith. No. 2 is a transverse section of hair of peccary, with a diversified arrangement of the cortical envelope, sending outward a set of radial prolongations and air-cells; this kind of structure is also found in the quills of the porcupine. No. 3 is a transverse section of a hair of the elephant, which shows a combination of a number of tubes united together, somewhat resembling the arrangement of the hoof-horn of some of the ruminants, and the denser horny growth on the snout of the rhinoceros, No. 4. The curious modification of these horny structures is seen in the horns of other animals, and which may be likened to a bundle of hairs. On making a transverse section, as in Fig. 434, and submitting it to polarised light, on rotating the analyser, the dark central spot shown is replaced by a bright one with a play of colours due to the interference of light (Plate VIII., No. 178). The scales of fish are also of interest (Fig. 435). These have been shown to afford an unerring guide in the classification of fishes and in the examination of their fossil remains. As a class of objects for the microscope, they are found to be both curious and beautiful. Plate VIII., No. 176, is a scale of the grayling, seen under polarised light.

Of the harder outgrowths of the dermal structures, the teeth afford the chief example among animals. The rough anatomy of the tooth in mankind consists of a crown, that projects from the gum; a root, or fangs, fixed in a socket of the jawbone, and a short intermediary neck. Each tooth is supplied with an artery and nerve, and has a central cavity filled with a soft, vascular, sensitive substance, the pulp. On making a vertical section of a tooth, we recognise the several structures in the order of, pulp, crusta, petrosa, dentine, and enamel. A section through a human molar tooth (shown in Fig. 436) will convey some idea of the arrangement of the denser structures referred to above.

Blandin was the first to demonstrate that teeth are developed in the mucous membrane, similar to that of hair and nails. Teeth are formed in grooves of the mucous membrane, and subsequently converted into closed sacs by a process of involution, and their final adhesion to the jaw is a later process. It is very generally conceded that teeth belong to the muco-dermoid, and not to the periosteal, series of tissues; that, instead of standing in close relation to the endo-skeleton, they are part of the dermal or exo-skeleton; their true analogues being the hair, and some other epidermic appendages. Huxley proved that, although teeth are developed in two ways, they are mere varieties of the usual mode in the animal kingdom. In the first, which is typified by the mackerel and the frog, the pulp is never free, but from the first is inclosed within the capsule, seeming to sink down as fast as it grows. In the other, the pulp projects freely at one period above the surface of the mucous membrane, becoming subsequently included within a capsule formed by the involution of the latter; this occurs in the human subject. The skate offers a sort of intermediate structure.

The enamel forms a continuous layer, and invests the crown of the tooth; it is thickest upon the masticating surface, and decreases towards the neck, where it usually terminates. The external surface of the enamel appears smooth, but is always marked by delicate elevations and transverse ridges, and covered by a fine membrane (Nasmyth’s membrane), containing calcareous matter. This membrane is separable after being subjected to hydrochloric acid; it then appears like a network of areolar tissue, shown in Fig. 438, No. 6; Huxley’s “calcified membrana,” which commence at the pulp cavity, and pass up to the enamel.

Czermak discovered that the curious appearances of globular conglomerate formations in the substance of dentine depend on its mode of calcification and the presence of earthy material; and he attributed the contour lines to the same cause. Contour markings vary in intensity and number; they are most abundant in the root, and most marked in the crown. Vertical sections exhibit them the best; as Fig. 440, No. 1. In preparing a specimen, first make the section accurately, then decalcify it by submersion in dilute hydrochloric acid; dry it and mount in Canada balsam; place the specimen in the hot chamber for some time to soak in the fluid resin before it cools. The white opacity at the extremity of the contour markings gives the appearance of rings to the tooth-fang.

“The tooth-substance appears,” says Czermak, “on its inner surface, not as a symmetrical whole, but consisting of balls of various diameter, which are fused together into a mass with one another in different degrees, and in which the dentine tubes in contact with the germ cavity terminate. By reflected light, dark-ground illumination, one perceives this stalactite-like condition of the inner surface of the tooth-substance very distinctly, by means of the varied illumination of the globular elevations, and by the shadows which they cast.” To see this structure to advantage the preparation should be made from a tooth root, the growth of which is not complete. With such preparations, the ground-substance of the last formed layer of the tooth-substance is seen to be, at least partly, in the form of globular masses, fused together with those of the penultimate layers.

The cementum is the cortical layer of osseous tissue, forming an outer coating to the fangs, which it sometimes cements together. Its internal surface is intimately united with the dentine, and in many teeth it would appear as if the earliest determined arrangement of the fibres of the dentine started from the canaliculi, as they radiate from the lacunÆ in the cement. The inter-lacunar layer is often striated, and exhibits a laminated structure: sometimes it appears as if Haversian canals were running in a perpendicular direction to the pulp cavity. The canaliculi frequently run out into numerous branches, connecting one with another, and anastomising with the ends of the dentine fibres. The thick layers of cement which occur in old teeth show immense quantities of aggregated lacunÆ of an irregular and elongated form.

Compact Tissues, Cartilage and Bone.—Cartilage is a bluish or yellowish-white, semi-transparent, elastic substance, without vessels or nerves, and surrounded by a membrane, termed pericondrium, of a dense fibrous nature. That kind, however, known as articular cartilage, receives a layer of epithelium from the synovial membrane, but this is confined to marginal portions, in consequence of the central wear which occurs as soon as the parts are subjected to friction, during the movement of the limbs. Cartilage covers the ends of all bones in apposition to form joints, and thus lessens the effects of concussion. Besides the ordinary kind of cartilage, temporary and permanent, there are two modifications of the tissue, confined to certain portions of the body: cellular cartilage, composed of cells lying close together, in a mesh formed of fine fibres; and fibro-cartilage, cells distributed in a matrix of fibrous tissue.

Examined with a low power, cartilage appears to be homogeneous in structure, studded over with numerous round, oval, oblong, semilunar, and irregular-shaped corpuscles, as seen in Plate XIX., No. 8, a vertical section of animal cartilage, arranged in columns, and condensed at the lower surface previous to its conversion into bone. The greater opacity of this portion is owing to the increase of osseous fibres, and the multiplication of oil globules, and the intercellular spaces becoming filled with vessels. No. 9 shows a small transverse section of the same, with a further change of the cartilage cells at a into bone cells, and at b with the characteristic canaliculi and lacunÆ. No. 7 further shows a section of the large tendon fixed to the back of the heel of the foot, near the juncture of the tendo-Archillis with the cartilage. For the examination of these several changes a high power is necessary, and for the purpose pieces taken from the ox may be easily obtained from the butcher, and fine sections cut with a razor parallel to the surface.

The better specimens for microscopical examination are those taken from very young animals, in whom the ossific process is still incomplete. In order to examine cellular cartilage, the ear of the mouse should be taken and just dried sufficiently to enable fine sections to be cut by the microtome transversely (Fig. 440).

Cartilage forms the entire skeleton of a certain number of fishes, as the skate, lamprey, ray, shark, &c., the cells of which are embedded in a matrix of granular matter, which has been properly termed intercellular. The nearest approach to ossification of cartilage in fishes is that of the cuttle-fish; in this stellate cells are freely distributed, as shown in Fig. 441, No. 3.

White fibro-cartilage occurs between the bodies of the vertebrÆ as a connecting medium. In this kind the cells are more widely distributed, specimens of which may be taken from the central portion of an interarticular disc of any animal. The oval or circular corpuscles will be seen surrounded by an abundance of fibrous tissue.

An acquaintance with the degeneration of the textures with which we have been dealing may be of service to the student, as he may, in the course of his examination, meet with an abnormal condition altogether different to those described. The process of degeneration is usually a slow one, except in the case of fatty infiltration, an example of which is furnished by the fatty degeneration of the liver in Strasburg geese. Muscular tissue is very prone to fatty degeneration, and fatty heart is often met with. Calcareous degeneration of the muscles, ligaments, and cartilages, as well as morbid deposits, are not at all uncommon in these structures. In Plate XIX., No. 9, a small section is given of an enchondroma, and in which the round or ovoid cells of the cartilage are seen degenerated and converted into granular masses of a calcareous nature. Fig. 442 is a somewhat more highly magnified section of a calcareous or morbid growth, taken from a human subject in which a morbid growth was seen to be gradually destroying the bone and cartilage cells.

Bone.—Bone is a hard unyielding structure, and which in the vertebrata forms the skeleton of the adult. It is the framework for the support of the soft tissues of the body, and forms various cavities for the reception of important organs, as the brain, spinal cord, eyes, heart and lungs, and acts as levers for the action of the muscles and joints. The partial elasticity of bone is seen in the ribs, and the rebound when the skull is dropped on the ground. Bone consists of earthy and animal matters intimately combined; the removal of either, however, does not destroy the form of the bone, if the process of separation be carefully conducted. The earthy constituents may all be dissolved out by hydrochloric acid, but the form of the bone is preserved in its minute particular, and in this state sections may be cut for microscopical examination. If allowed to become dry it shrivels, and assumes the density of horn. The interior of a bone is of a spongy or cancellated structure, particularly at the ends. The outer portion of the bone is more dense than the internal part. The study of bone should commence with sections of the softened structure. Directions for making sections of bone are given in the chapter on Practical Microscopy.

The intimate structure of bone will be studied in connection with Plate XX. Two series of lamellÆ may be demonstrated in bone after maceration in acid, a larger system surrounding the medullary canal, and a smaller surrounding the Haversian canals, both of which are seen in Nos. 1 and 2. In macerating bones, the lamellÆ of the layer concentric system may be peeled off in layers; these are seen to be pierced with fine apertures, caused by the canaliculi. In some parts larger apertures are seen through which bundles of fibres pass, pinning, as it were, the several layers together; these are the perforating fibres. The outermost of the layers, being near the periosteum, the membrane covering the bone, are termed periosteal layers; the innermost, being close to the canal, are called medullary layers. No. 1 is a transverse section of a flat bone, the clavicle, and it shows the Haversian canals, varying in size from ¹/₂₀₀₀th to ¹/₂₀₀th of an inch in diameter, the largest being near the medullary canal. In shape they are round, oval, or oblong, according to the line of section. Each canal is surrounded by rings, none of which are complete, and running one into the other at various parts. Under a higher power, those irregular shaped bodies termed lacunÆ, with fine radiating fibres, are seen to be smaller canals, canaliculi.

By means of this complete and intricate distribution of the canals of the Haversian system, the nutritive fluids pass into the most compact parts of the osseous tissue. Longitudinal sections of the long bones show these canals as continuous branching-out cells.

In many of the lower animals the bony structure differs from those of man, as will be seen in Plate XX. No. 3 shows a transverse section of the femur, or leg-bone of an ostrich, magnified ninety-five times, in which the Haversian canals are much smaller and more numerous, and many of them run in the transverse direction. No. 4, again, is a transverse section of the humerus, or fore-arm bone of a turtle (Chelonia mydas). This exhibits traces of Haversian canals, with a slight tendency to a concentric arrangement of bone-cells around them, the bone-cells being large and numerous, and occur, for the most part, in parallel rows. In No. 5, a horizontal section of the lower jaw-bone of a conger-eel exhibits a single plane of bone-cells arranged in parallel lines. There are no Haversian canals present, and when this specimen is contrasted with that of No. 4, it will be noticed that the canaliculi given off from each of the bone-cells of this fish are very few in number in comparison with that of the reptile. No. 6 is a section of a portion of the cranium of a siren (Siren lacertina), remarkable for the large size of the bone-cells, and of the canaliculi, which are larger in this animal than in any other yet examined; and as in the preceding specimen, no Haversian canals are present. No. 7 is a section of bone taken from the exterior of the shaft of the humerus of a Pterodactyle; this exhibits the elongated bone-cells characteristic of the order Reptilia. No. 8 is a horizontal section of a scale, or flattened spine, from the skin of a Trygon, or sting ray; this exhibits large Haversian canals, with numerous wavy parallel tubes, like those of dentine, communicating with them. This specimen shows, besides wavy tubes, numerous bone-cells, whose canaliculi communicate with the tubes, as in dentine.

The following points may be noted with regard to the several sections of bone described. That of the bird, for instance, contrasted with that of the mammal, exhibits the following peculiarities: the Haversian canals are more abundant, much smaller, and often run in a direction at right angles to that of the shaft, by which means the concentric laminated arrangement is in some cases lost; the direction of the canals follows the curve of the bone; the bone-cells are much smaller and more numerous; while the number of canaliculi sent off from the cells is less than in those of mammals. No. 3 is the average length of a bone-cell of the ostrich, ¹/₂₀₀₀th of an inch, in breadth ¹/₆₀₀₀th.

In the Reptilia, the bones may be either hollow, cancellated, or solid; and their specific gravity is less than that of birds or mammals. The short bones of most of the chelonian reptiles are solid, and the long bones are either hollow or cancellated; the ribs of the serpent-tribe are hollow, the medullary cavity performing the office of a Haversian canal; the bone-cells are accordingly arranged in concentric circles around their canals. The vertebrÆ of these animals are solid; and the bone, like that of certain birds, is remarkable for density and whiteness. When a transverse section is taken from one of the long bones, and contrasted with that of a mammal or bird, the difference will be noticed; there are very few, if any, Haversian canals, and these are large; and at one view, in the section, No. 7, the canals and bone-cells are arranged both vertically and longitudinally. The bone-cells are remarkable for the great size to which they attain; in the turtle they are ¹/₃₇₅th of an inch in length, the canaliculi are extremely numerous, and are of a size proportionate to that of the bone-cell.

In fishes a greater variation occurs in the minute structure of the skeleton than in either of the three preceding classes. A rare structure is that of the sword of the sword-fish (Istiophorus). In this, Haversian canals and a concentric laminated arrangement of the bone are found, but no bone-cells. The Haversian canals, when they are present, are of large size, and very numerous, and then the bone-cells are, generally speaking, either absent or but few in number, their place being occupied by tubes or canaliculi, which are often of a very large size. The bone-cells are remarkable for their graduate figure, and the canaliculi derived from them are comparatively few in number. In a thin section of the scale of an osseous fish, the cells lie nearly all in one plane, and the anastomoses of the canaliculi are more distinctly seen; in the hard scales of many, as the Lepidosteus and Calicthys, and in spines of the SiluridÆ, the bone-cells are well differentiated. In the true bony scales comprising the exo-skeleton of cartilaginous fishes the bone-cells are seen in great numbers.

Now, if we proceed at once to the application of the facts which have been laid down, and make a fragment of bone of an extinct animal the subject of investigation, it will be found that the bone-cells in Mammalia are tolerably uniform in size; and if we take ¹/₂₀₀₀th of an inch as a standard, the bone-cells of birds fall below that standard; but the bone-cells of reptiles are much above either of the two preceding, while those of fishes are essentially different, both in size and shape, and are not likely to be mistaken for one or the other; so that the determination of a minute yet characteristic fragment of fishes’ bone is a task easily performed. If the portion of bone does not exhibit bone-cells, but presents either one or other of the characters indicated, the task of discrimination is equally easy. We have now the mammal, the bird, and the reptile to deal with. In consequence of the very great size of the cells and their canaliculi in the reptile, a portion of bone of one of these animals can readily be distinguished from that of a bird, or a mammal. The only difficulty lies between these two last; but, notwithstanding that on a cursory glance the bone of a bird appears very like that of a mammal, there are certain points in their minute structure in which they differ; and one is the difference in size of the bone-cells. To determine accurately, therefore, between the two, we must, if the section be a transverse one, also note the comparative sizes of the Haversian canals, and the tortuosity of their course; for the diameter of the canal bears a certain proportion to the size of the bone-cells, and after close examination the eye will readily detect differences.

Arteries and Veins.—The circulation of the animal frame is maintained by arteries, veins, and capillaries. The arteries are elastic and contractile tubes; these convey the blood from the heart to the capillaries. The larger arteries are exceedingly elastic, but feebly contractile on account of the muscular tissue in their walls. The veins ramify throughout the body, are more numerous than the arteries, and of greater capacity. They usually accompany the arteries and correspond to them in structure, the larger veins possessing semi-lunar valves; these project into their interiors, and thus prevent the regurgitation of the blood. They have four coats, consisting of areolar tissue, yellow fibres combined with muscular fibres, and white fibrous tissue, two layers of yellow fibres arranged longitudinally, and a single layer of epithelial cells. Intermediate between the arteries and veins there are exceedingly fine tubes, termed capillaries, in which the arteries terminate, and from which the veins arise. These are composed of a fine homogeneous membrane, with here and there a nucleus. The capillary circulation of the blood is readily seen in the tail of the newt and the foot of the frog, Fig. 443.

A network of capillaries conveying blood to the lungs, and ramifying throughout the structure, is shown in Fig. 444, and in Plate XIX., No. 6, the termination of a capillary of a blood-vessel in the fat-cells of the human body. Plate VII. illustrates the distribution of the arteries and veins to various parts of the animal body. This coloured plate, however, is designed to show the value of injected preparations in the delineation of animal structures. By thus artificially restoring the blood and distending the tissues, a much better idea is obtained of the relative condition of parts, the appearance presented by the erectile papillÆ, &c. In the section of foot of mouse (No. 149), the bone is seen surrounded by its vascular supply, arterial and venous; in No. 150, the papillÆ of the tongue are distended and seen erect; in No. 152, a vertical section of the fungi-form papillÆ on the tongue of cat, with capillary loops passing into them, is demonstrated; in No. 151, the vertical section of brain of a rat, the vascular supply is shown; No. 153, the malpighian tufts (circular bodies) and arteries ramifying about the structure; in No. 154, the vertical section through the intestine of the rat, shows villi (arteries and veins) surmounted by epithelium, and supported on a layer of the mucous membrane; in No. 155, the vascular supply sent to the roots of the whisker of the nose of the mouse; in No. 157, a tangential section cut through the several textures, the sclerotic coat and retina of the eye of a cat is clearly made out although not highly magnified; again, in No. 156, the beautiful vascular arrangement of the internal gill of the tadpole could scarcely be so strikingly illustrated in any other way; while in the central, No. 158, the vascular system throughout the whole of the body of a fully developed tadpole, with the way in which the blood is carried from the remotest part of the tail to the heart, and sent to the gills, the brain, &c., it is quite unnecessary to enlarge upon. These are seen under a low power, but for the purpose of studying the basement membrane, together with the intimate association and termination of the nerves accompanying the arteries and veins, it is absolutely necessary to resort to a staining process, and cutting fine sections with the microtome. Small portions of a nerve may be cut off with fine scissors, teased out with needles, and a drop of acetic acid added to render the sheath more transparent; in a few seconds the connective tissue corpuscles will be brought into view. For the microscopical examination of nerve-fibrillÆ take a small section from the leg of a frog, and tease it out in blood serum or white of egg. In size the fibrillÆ vary, even in the same nerve, from the ¹/₁₂₀₀₀th to the ¹/₁₅₀₀th of an inch in diameter.

To show the circulation of the blood in the frog’s foot, and without causing the animal pain or much inconvenience, it is better to enclose it in a black silk bag, and draw out the foot as shown at a a a, Fig. 445. The bag provided should be from three to four inches in length, and two and a half inches broad, shown at b b, having a piece of tape, c c, sewn to each side, about midway between the mouth and the bottom, and the mouth itself capable of being closed by a drawing-in string, d d. Into this bag the frog is placed, and only the leg which is about to be examined kept outside; the string d d must then be drawn sufficiently tight around the small part of the leg to prevent the foot from being pulled into the bag, but not to stop the circulation; three short pieces of thread, f f f, are now passed around the three principal toes; and the bag with the frog must be fastened to the plate a a by means of the tapes c c. When this is accomplished, the threads f f f are passed either through some of the holes in the edge of the plate, three of which are shown at g g g, in order to keep the web open; or, what answers better, in a series of pegs of the shape represented by h, each having a slit, i, extending more than halfway down it; the threads are wound round these two or three times, and then the end is secured by putting it into the slit i. The plate is now ready to be adapted to the stage of the microscope: the square opening over which the foot is secured must be brought over the aperture in the stage through which the light passes from the mirror.

The tadpole circulation is readily seen by placing the creature on its back, when we immediately observe the beating heart, a bulbous-looking cavity, formed of delicate, transparent tissue, through which the blood alternately enters by one orifice and leaves by a more distant exit. The heart, it will be noticed, is enclosed within its pericardium, this being the more delicate part of the creature’s organisation. The binocular microscope should be used for viewing the circulation. Passing along the course of the great blood-vessels to the right and left of the heart, the eye is arrested by a large oval body, of a more complicated structure. This is the inner gill, formed of delicate, transparent tissue, traversed by arteries, and a network of blood-vessels. It is almost unnecessary to say the tadpole has a respiratory and circulatory system resembling those of fishes.

In nearly all fish the heart has but two cavities, an auricle and ventricle; the blood is returned by the veins to the auricle, passes into the ventricle, and is then transmitted to the gills, where, being exposed to the air contained in the water, it becomes deprived of carbonic acid, aerated, and rendered fit to breathe. In the reptile we find a modification of plan. The heart has three cavities, two auricles and one ventricle; by this contrivance there is a perpetual mixture in the heart of the impure carbonized blood which has already circulated through the body, and flows into the ventricle from the right auricle, with the purer aerated blood returned from the lungs, which flows at the same instant into the ventricle from the left auricle.

For the purpose of subsequent observations the tadpole should be selected at a period in which the skin is perfectly transparent, otherwise the appearances already described of the form and situation of the heart, and the three great arterial trunks (proceeding right and left), will not be clearly made out. The anatomical arrangement of the vessels will be seen to be closely connected with the corresponding gill, the upper one (the cephalic) running along the upper edge of the gill, giving off, in its course, a branch which ascends to the mouth, with its accompanying vein; this is termed the labial artery and vein. The cephalic artery continues its course around the gill, until it suddenly curves upwards and backwards, and reaches the upper surface of the head, when it dips down between the eye and the brain.

It must not be supposed that this can be made out in the average tadpole, the obstacle to which is the large coil of intestines, usually distended with dark-coloured food. This must first be reduced by making your tadpole live on plain water for some days. Plate VII., No. 158, affords a view of the vessels obtained under the influence of low diet, and whereby we are enabled to trace the course of the three large arteries. The third trunk, traversing the lung, is seen to emerge from the lower edge and descend into the abdomen to form the great abdominal aorta. A small half-starved tadpole shows the heart beating and the blood circulating, but the latter is quite colourless, not a single red globule visible anywhere. The heart is a colourless globe, the gills two transparent ovals, and the intestines a colourless, transparent coil. Through the empty coil the artery is seen on either side leaving the gills, and converging towards the spine, and uniting to form the abdominal aorta, the large central vessel coloured red in the figure. After the aorta has supplied the abdominal viscera, a prolongation, or caudal artery is seen descending to the tail, the all-important organ of locomotion in the tadpole. This artery, entering the root of the tail, is imbedded deeply in the flesh, whence it emerges, and then continues its course, closely accompanied by the vein, to within a short distance of the extremity, where, being reduced to a state of extreme fineness, it terminates in a capillary loop, composed of the end of the artery and the beginning of the vein. The artery, in its course, gives off branches continually to supply the neighbouring tissue. The blood-current in the tail is often seen, even in the main artery or vein, to be sluggish. This occurs independently of the heart, which will continue to beat as usual; it happens, because the circulation in the tail depends very much on the motion of the organ. When this is suspended (as in the confined tadpole under the microscope), the blood moves sluggishly, or stops, till the tail regains its freedom and motion, when the activity of the current is restored.

Having traced the arterial system which conveys the blood from the heart to the extremities, we will now note its return by the veins back again to the heart.

The caudal vein runs near the artery during the greater part of its course, with its stream of blood towards the heart. This stream is swollen by perpetual tributaries from numerous vessels. As the vein approaches the root of the tail it is inclined towards the artery, and diverges from it at the point of entering the abdomen. Here it approaches the kidneys and sends off branches, while the main trunk continues its course onward; and, passing upwards behind a coil of intestine, it approaches the liver, and runs in a curved course along the margin of that organ. The blood is now seen to enter the vena cava by several channels, that converge towards the great vein as it passes in close proximity to the organ. Beyond the liver the vena cava continues its course upwards and inwards to its termination in the sinus venosus or rudimentary auricle of the heart. This termination is the junction of not less than six distinct venous trunks, incessantly pouring their blood into the heart. The circulation in the fringed lips forms a most complicated network of vessels, out of which proceeds a vein corresponding to the artery already traced. This descends in a direct course till it joins the principal vein of the head, which corresponds to the jugular in the mammalia.

Thus it will be seen the blood is driven by the heart into each inner gill through three large blood-vessels, which arise directly from the truncus arteriosus, and may be called the afferent vessels of the gill. In Plate VII., No. 156, an enlarged view of a gill is shown.

On closer examination “each internal gill or entire branchial organ is seen to consist of cartilaginous arches, with a piece of additional framework of a triangular form, stretching beyond the arches, composed of semi-transparent, gelatinous-looking material. These form the framework of the organ and support upon their upper surface the three rows of crests with their vascular network, and the main arterial and venous trunks lying parallel to and between them. The three systemic arteries arising, right and left, from the truncus arteriosus, enter each gill on its cardiac side, and then follow the course of the crests, lying in close proximity to them. The upper of these branchial arteries runs alone on the outside of the upper crest, and another branch leaving the trunk and passing into the network of the crest, whence a returning vessel may be traced carrying back the blood across the branchial artery, and to a vessel lying close to and taking the same course as the artery itself. Carrying the eye along the latter vessel we find, at a short distance from the first of these crest branches, a second, leaving the main trunk and entering the crest, when a corresponding returning vessel conveys the blood across the arterial trunk into the vessel lying beside it, as in the former instance. A number of these branches may be traced from one crest to the other. But it is now seen that the trunk from which these arterial branches spring diminishes in size as it proceeds in its course (like the gill artery in fishes), while the vessel running parallel to it and receiving the stream as it returns from the crest enlarges to some extent. Thus, the artery or afferent vessel which brings the blood to the gill is large at its entrance, but gradually diminishes and dwindles to a point at the opposite end of the crest; while the venous or efferent vessel, beginning as a mere radical, gradually enlarges, and thus becomes the trunk that conveys the blood out of the gill to its ultimate destination. This vessel is the upper branchial vein so long as it remains in contact with the gill; subsequently it changes its name on leaving the gill and as it passes upwards for distribution to the head, when it is designated the cephalic artery. The middle branchial artery and vein proceed in like manner in connection with the middle crest, and the lower artery and vein in connection with the lower crest. The middle and lower venous trunks, having reached the extremity of the crests, curve downwards and inwards, and leave the gill. The former trunk, converging towards the spine, meets its fellow, and with it forms the ventral aorta. The latter gives origin to the pulmonary artery, and supplies also the integuments of the neck. Curious and interesting is the final stage of the metamorphosis, when the waning tadpole and incipient frog coexist, and are actually seen together in the same subject. The dwindling gills and the shrinking tail—the last remnants of the tadpole form—are yet seen, in company with the coloured, spotted skin, the newly formed and slender legs, the flat head, the wide and toothless mouth, and the crouching attitude of the all but perfect reptile.”87

To observe the circulation and how it is carried on during life in the gills, the outer covering must be carefully raised, or even stripped off. This will be better accomplished by putting the tadpole under the influence of cocaine or chloroform—a drop of the fluid is sufficient for the purpose.

The metamorphosis in the embryo of the frog is by no means exceptional. The ascidian begins life in the form of a tadpole, with a muscular tail; subsequently it fixes itself by its head to a rock, and its tail disappears. The changes the tadpole of the frog passes through are in every respect, except in one or two minor details, similar to those of adult amphibia which pass their whole lives in water. The newly-hatched flat-fish is symmetrical, an eye being placed on each side of its head, with the adults of other fishes. The foetal whale has well-developed hind limbs, and which, after passing into a condition almost perfect in proportion to the rest of the body, gradually dwindle away again to the merest rudimentary structures. In all these, and a number of similar cases, it is seen that the earlier condition of existing animals represents, and is in agreement with that of its adult ancestor of a remote period in the past. Collected facts bearing upon this question have been made the groundwork of a theory of hereditary properties in the germ, and a disposition to go through the same phases of life as the parent.