Nature of the function—Why involved in obscurity—Basis of the apparatus consists of membrane—Arrangement of membrane into elementary secreting bodies—CryptÆ, follicles, cÆca and tubuli—Primary combinations of elementary bodies to form compound organs—Relation of the primary secreting organs to the blood-vessels and nerves—Glands simple and compound—Their structure and office—Development of glands from their simplest form in the lowest animals to their most complex form in the highest animals—Development in the embryo—Number and distribution of the secreting organs—How secreting organs act upon the blood—Degree in which the products of secretion agree with, and differ from, the blood—Modes in which modifications of the secreting apparatus influence the products of secretion—Vital agent by which the function is controlled—Physical agent by which it is effected.

708. Secretion is the function by which a substance, gaseous, liquid, or solid, is separated or formed from the nutritive fluid. It is a function as necessary to the plant as to the animal, and indispensable alike to the life of both. It is of equal importance to the preservation of the individual and to the perpetuation of the species. In all living beings secretions are separated from the nutritive fluid, and added to the aliment to assist in converting it into nutriment, and are separated from the nutriment to maintain the composition of the nutritive mass in a state fit for the continued performance of the act of nutrition, and to form the germ on the development of which the continuance of the species depends.709. The secretions of the plant, varied and abundant, are indispensable to its nourishment, growth, and fructification. The secretions of the animal more diversified, and far more constantly performed, increase in number and elaborateness in proportion to the range and intensity of the vital endowments and actions. In all animals high in the scale of organization, and especially in man, the products of secretion are vast in number, and exceedingly complex in nature,—membrane, muscle, brain, bone;—the skin, the fat, the nail, the hair;—water, milk, bile, wax, saliva, gastric juice;—whatever substances enter as constituents into the corporeal structure;—whatever substances are specially produced, in order to perform some definite purpose in the economy;—whatever substances are separated from the mass, and carried out of the system on account of their useless or noxious properties:—all are derived from the nutritive fluid, the blood, and are formed from it by the process of secretion.710. In this function are included the most secret and subtle processes of the vital economy,—the ultimate actions of the organic life. Of the real nature of those actions nothing definite is known; and they are modified by agencies over which the art and skill of the experimentalist can exert no adequate control. It is not wonderful therefore that they should be involved in obscurity: nevertheless, when all the phenomena are collected and compared, much of the mysteriousness in which the function appears at first view to be involved vanishes.711. The apparatus of secretion is infinity varied in form: when examined in its complex combinations it appears inextricable in structure, but the diligence and skill of modern research have unfolded much of its mechanism, and enabled us to trace the successive steps by which it passes from its simple to its complex condition.712. To form an organ of secretion there must be an artery, a vein, a nerve, an absorbent, and a sufficient quantity of cellular tissue to allow of the free expansion of these vessels and of their complete intercommunication. Membrane constitutes such an organ; for membrane is composed of arteries, veins, nerves, and absorbents sustained and connected by cellular tissue. Hence membrane constitutes a secreting organ, in its simplest form. The most important secreting membranes are the serous (30), the cutaneous (34), and the mucous (33).713. Serous membrane which lines the great cavities of the body, and which gives an external covering to the organs contained in them (fig. LX. a, c), forms an extensive secreting surface. Synovial membrane, or that which covers the internal surface of joints, and which constitutes an important portion of the apparatus of locomotion, is essentially the same in structure and office.714. Cutaneous membrane, or the skin, which forms the external covering of the body, is an organ in which manifold secretions are constantly elaborated; but the skin is only a modification of the membrane which lines the interior of the body, the mucous. Mucous membrane forms the basis of the secreting apparatus placed in the mouth, fauces, esophagus, stomach, and intestines in their whole extent; of the secreting apparatus auxiliary to that of the alimentary canal, namely, the pancreas and the liver; probably also of the mesenteric, or lacteal glands, together with the vast system of lymphatic glands, and certainly of the glands of the larynx, trachea, bronchi and air vesicles of the lungs. Hence, while membrane forms the basis of the secreting apparatus in general, mucous membrane is far more extensively employed in its construction than any other form of membrane.715. 1. In the construction of the secreting apparatus, membrane disposed in the simplest form, constitutes merely a uniform, smooth, extended surface. Serous membrane is always disposed in this simple mode. The costal pleura which lines the internal surface of the walls of the chest (fig. LX. a); the pulmonary pleura which is continued from the walls of the chest over the lungs (fig. LX. 5); the peritoneum which lines the internal surface of the cavity of the abdomen, and which is reflected over the viscera contained in it (fig. LX. c, and 6, 7, 8, &c.); the synovial membrane which covers all the articular surfaces; the arachnoid membrane which envelopes the brain, form simple continuous, serous, secreting surfaces. On the contrary, mucous membrane is never disposed in this perfectly simple mode; even when it forms a continuous surface, as in the lining, which it affords to the alimentary canals, it is more or less plaited into folds or rugÆ (fig. CLXVII. 1).

716. 2. The second disposition of membrane in the construction of the secreting apparatus, is the depression of it into a minute pit or fova, called a crypt (CLXXXI. ), which is sometimes inclosed on all sides, forming a cell or vesicle (fig. CXXXVIII.).

717. 3. Next, the vesicle, instead of being rounded, is elongated into a peduncle or neck, not unlike the neck of a bottle (fig. CLXXXII. 1). This pedunculated vesicle is called a follicle.718. 4. Then, the follicle is somewhat elongated, without neck and without terminal expansion (fig. CLXXXVI. 1); and this is called a cÆcum or pouch.719. 5. And, lastly, the cÆcum itself is elongated; so that instead of presenting the appearance of a pouch, it rather resembles a tube (fig. CLXXXV. 1), and is accordingly named tubulum.720. In the construction of the secreting apparatus, membrane, then, may be said to be disposed into four elementary forms constituting cryptÆ or vesicles, follicles, cÆca and tubuli. Membrane, disposed into these elementary forms, constitutes the simple bodies by the accumulation and the varied arrangement of which the compound organs are composed. There is no other known element which enters into the composition of the most complex secreting organ.721. One of these elementary bodies may exist as a simple organ, or many may be collected into a mass to form a compound organ. When single they are called solitary: when collected into a mass, aggregated. Each elementary body has a mode of aggregation peculiar to itself. Vesicles aggregate by clustering together (fig. CXXXVIII.), and adhering as if by a common stem (fig. CXXXVIII.); follicles by uniting at their orifices (fig. CLXXXIII.), and forming masses which are disposed either in a linear direction (fig. CLXXXIII.) or in fasciculi (fig. CLXXXIV.); cÆca by forming bundles, parallel or branched (fig. CLXXXVI.); and tubuli by forming masses straight (fig. CLXXXV.), tortuous or convoluted (figs. CLXXXV. and CLXXXIX.).

722. When a single elementary body, as a vesicle or follicle, forms a distinct secreting organ, the matter secreted is elaborated at the inner surface of the organ (fig. CLXXXII. 2), and is contained within its cavity. When needed it quits this cavity through the walls of the vesicle, or at the orifice of the follicle, on the application of the appropriate stimulus. When a number of cryptÆ or vesicles are aggregated into clusters, the individual vesicles sometimes open by distinct orifices into a common receptacle or sac (fig. CLXXXIV.). When follicles are aggregated into a mass, and the mass is disposed in a linear direction (fig. CLXXXIII.), each follicle pours out its secreted matter by its own orifice (fig. CLXXXIII.); but if conglomerated, into a common mass by a common orifice (fig. CLXXXIV.).

723. In like manner, in some very simple arrangements of cÆca and tubuli, each body opens by its own distinct orifice (fig. CLXXXV. 2). But in the more complex arrangements of these bodies, it is indispensably necessary to modify this mode of parting with their contents. When the elementary bodies are aggregated into dense, thick masses (fig. CLXXXIX.), when layer after layer of these masses containing myriads of myriads of follicles, cÆca, or tubuli, are superimposed one upon another, (fig. CLXXXIX.), it is impossible that each individual body can have a separate orifice. In this case a minute tube springs from each body (fig. CLXXXVI. 2); and a complete connexion is established between all the individuals composing the mass by the free intercommunication of these tubes (fig. CLXXXVI. 2). Of these tubes the minutest unite together, and form larger branches (fig. CLXXXVI. 2); these larger branches again uniting form still larger branches (fig. CLXXXVI. 2), until, by their successive union, the branches form at length a single trunk (fig. CLXXXVI. 3), with which all the individual branches, whether great or small, communicate, and into which they all pour their contents (fig. CLXXXII. 2, 3). The bodies from which these tubes take their origin, and the minute tubes themselves, are called secreting canals (fig. CLXXXII. 1, 2); the common trunk formed by their union is termed the excretory duct (fig. CLXXXII. 3). The secreting canals contain the secreted matter; the excretory duct collects this matter, and conveys it to the part of the body in which it is appropriated to the specific purpose which it serves in the economy.724. The basis of the secreting canals consists, then, of membrane disposed in one or other of the elementary forms described (712, et seq.), These secreting canals constitute a peculiar system of organs wholly different from all the other organs of the body. The form of these organs, their structure and their relation to the blood-vessels and nerves, have formed subjects of laborious investigation and of keen controversy during several centuries. The honour of discovering the exact truth on these points is due to very recent researches.725. Malpighi, an Italian, who flourished at Bologna in the middle of the 17th century, was the first to establish a special inquiry into the intimate structure of the secreting apparatus. After many years of laborious examination he arrived at the conclusion that a minute sac or follicle is invariably interposed between the termination of the capillary artery and the commencement of the excretory duct. According to him, the capillary artery conveys the blood to the follicle, separates from the blood the substance secreted, and the excretory duct arising from one extremity of the follicle conveys the secreted fluid, when duly prepared, to its destined situation. By injection, by dissection, by the microscope, by experiment on living animals, and by the phenomena of disease, he conceived that he had demonstrated that this is the true structure of the secreting apparatus in its most complex form. This view was generally acquiesced in by his contemporaries and by succeeding anatomists and physiologists; and in the time when Ruysh wrote was the received opinion.726. Ruysh, who flourished at Amsterdam, and was contemporary with Malpighi, but a younger man, and who published on the glands about twenty years after Malpighi, according to the account of Haller, “employed wonderful patience, with the assistance of his daughters, in rendering all his preparations elegant and beautiful, being equally skilled in the methods of softening, hardening, filling, and drying.” Of Ruysh it was said that while others, in their anatomical preparations, merely exhibited the horrid features of death, he preserved the human body in all the freshness of life, even to the expression of the features. The fineness of his injections, the dexterity with which he unfolded the minute vessels, nerves, and absorbents, and exhibited their combinations and relations in the most delicate structures, the skill with which he preserved his preparations in transparent fluids, and the elegance with which he displayed them in their natural forms and folds, excited universal admiration; and philosophers, statesmen, princes, kings, all the learned and noble of the day, crowded to his museum.727. By his superior method of injecting, Ruysh conceived that he was able completely to disprove Malpighi’s doctrine. He maintained that the bodies which Malpighi mistook for sacs or follicles are in reality convoluted vessels; that these vessels are capable of being completely unravelled; that, when unfolded, their continuity with the excretory duct is perfectly demonstrated; that secretion is performed by the capillary artery itself, without the intervention of any other organ; and that when the secreted substance is duly prepared, it is poured by the capillary directly into the excretory duct.728. Modern research has demonstrated that the opinion of Malpighi approaches nearer the truth than that of Ruysh, who appears to have mistaken the secreting canals for the ultimate division of the arterial vessels. Malpighi, indeed, did not succeed in discovering the elementary bodies of which the secreting apparatus is composed; but he arrived at the very verge of the truth. Profiting by the art which Ruysh brought to so much perfection, by the facts which Malpighi disclosed, and, above all, by the improved structure of the microscope, and the increased skill which has been acquired in the manipulation of the instrument, the modern physiologist is enabled to see what was formerly beyond the cognizance of sense, and to demonstrate what before could only be matter of conjecture. Availing himself of these advantages with consummate skill, and applying himself to the task with indefatigable industry, Professor MÜller, of Berlin, has investigated the structure of the secreting apparatus in the whole animal kingdom, and has traced the progressive development of the several secreting organs through the entire animal series, from their simplest form in the lowest animal, to their most complex in the highest.729. From the researches of this physiologist, and from the labours of others, his countrymen and contemporaries, who have engaged in the investigation with an ardour second only to his own, it is demonstrated that the secreting apparatus of the animal body is disposed in one or other of the elementary forms which have been described. The blood-vessels are distributed upon the walls of these elementary bodies, whether simple cryptÆ follicles, cÆca, or tubuli, or whether these bodies are accumulated and combined into the largest and most complex series of secreting canals, just as the branches of the pulmonary artery are distributed upon the walls of the air-vesicles in the rete mirabile of the lungs. The air-vesicles of the lungs are secreting organs, and afford an excellent example of the mode in which the blood-vessels are distributed upon the walls of the elementary secreting bodies. The arteries do not form continuous tubes with the secreting bodies or their excretory ducts, as was maintained by Ruysh; neither is the secreting body interposed between the termination of the artery and the commencement of the excretory duct, as was thought by Malpighi; but the ultimate divisions of the arteries are spread out upon the walls of the secreting bodies, where they terminate in veins by a delicate vascular net-work (fig. CLXXXVII. 2). The minutest branch of the artery is always smaller than the minutest secreting body on the walls of which it is distributed. According to MÜller, the arteries, spread out upon the walls of the secreting bodies, form a distinct and peculiar system of vessels visible under the microscope. In the more complex secreting organs, immediately before reaching their distribution upon the walls of the secreting canals, the ultimate divisions of the arteries form an intricate and delicate net-work (fig. CLXXXVII. 2). When at length they reach the secreting canals the arteries no longer divide and subdivide, but are always of the same uniform size in the same secreting organ, though their magnitude is different in every different kind of secreting organ. These ultimate divisions of the arteries are the proper capillary arteries. It is in these arteries that the changes are wrought upon the blood which it is the object of the various processes of secretion to effect. In the walls of these arteries there are visible no pores, no apertures, no open extremities by which the secreted fluid, when formed from the blood, is conveyed into the cavity of the secreting canals; it probably passes through the walls of the vessels into the secreting canals by the process of endosmose (804).

730. Secreting organs are very abundantly supplied with nerves, which are derived for the most part from the organic portion of the nervous system; although for the reasons assigned (vol. i. p. 77, et seq.) sentient nerves are mixed with the organic. The more important secreting organs have each a distinct net-work or plexus of organic nerves, which surround the blood-vessels distributed to the organ, (fig. CLXX. 3), and which envelopes more especially the arterial trunks and their larger branches (fig. CLXX. 3). From these plexuses nervous filaments spring in countless numbers (fig. CLXX. 3), which are spread out upon the walls of the arteries, just as the arteries are spread out upon the walls of the secreting canals. The nerves never quit the arteries; are never spent upon the membranous matter which forms the basis of the secreting organ, but are lost upon the walls of the capillary arteries. The nerves uniformly increase in number and size as the arteries diminish in magnitude and as their capillary terminations become thinner and thinner.731. When the secreting apparatus consists of simply extended membrane, a close net-work of capillary arteries with their accompanying nerves is spread out over the whole extent of the secreting surface. This simple arrangement is sufficient to separate from the blood the simple secretion in this case required.732. When the secreting apparatus consists of simple cryptÆ, follicles, cÆca, or tubuli, a similar net-work of capillary arteries and nerves is spread out on the sides of this more extended surface. The more elaborate secretion now formed is received into the interior of these organs, where it remains for some time, and whence it is ultimately conveyed as it is needed by the actions of the system.733. But when the secreting apparatus consists of aggregates of cryptÆ, follicles, cÆca, and tubuli, with their net-works of arteries and nerves, a much more complex structure is built up, which is destined to perform a proportionably elaborate function. An aggregation of these secreting bodies into a large mass, enveloped in a common membrane, so as to form a distinct body of a solid consistence, constitutes the organ termed a gland. Simply extended membrane, with its apparatus of arteries and nerves does not constitute a gland. Simple cryptÆ, follicles, cÆca, and tubuli, with their larger apparatus of arteries and nerves, do not constitute a gland. The first is simply secreting surface; the second are simply secreting cryptÆ, follicles, cÆca or tubuli; but when these bodies are aggregated into dense and solid masses with an extended system of excretory ducts, and when the whole of this apparatus is inclosed in a proper membrane so as to form a distinct body, such a body is termed a gland.734. Primary aggregations of these secreting bodies constitute what is termed a conglobate, that is, a simple gland; such are all the glands connected with the absorbent or lymphatic system. Secondary aggregates, or aggregates composed of simple glands, constitute what is termed a conglomerate, that is, a compound gland; such are all the organs commonly termed viscera, as the liver, the spleen, the pancreas, the kidney, and so on.735. The conglobate, or simple gland, being formed by the aggregation of cryptÆ, follicles, cÆca, or tubuli, inclosed in a proper membrane, presents the appearance of a simple solid body, commonly of a rounded or oblong form (fig. CLXXVI. 516). On the contrary, the conglomerate or compound gland, being formed by the aggregation of conglobate or simple glands, presents the appearance of a compound body composed of a congeries of masses (fig. CLXV. 1). The larger masses enveloped in their own proper membrane are termed lobes (fig. CXCI.); the smaller masses, also enveloped in their own proper membrane, are termed lobules (fig. CXCI.); the lobules, when carefully examined, are seen to be composed of still smaller masses, and these of masses yet more minute, until at length patient, laborious, and skilful dissection brings into view the ultimate constituent elements, which are invariably found to consist of simple cryptÆ, follicles, cÆca, or tubuli.736. Thus membrane having a specific arrangement of blood-vessels and nerves, from being simply extended, is folded into a few elementary forms; the bodies which result constitute simple secreting organs; these bodies collected together form, by their aggregation, compound organs; the compound organs, uniting, form aggregates still more compound, until at length a structure is built up highly elaborate and complex. But this complexity of combination and arrangement does not alter the constitution of the organs; their form varies, but their nature remains essentially the same. All consist alike of membrane organized in a similar mode. The complex contains no element not possessed by the simple gland, and the gland contains no element not possessed by the secreting surface. But there is this difference in the complex organs. Every kind and degree of change in the form of the secreting apparatus, from membrane simply extended, to membrane coiled up into the most complex gland, is attended with an accumulation and concentration of secreting surface. The crypt contains a larger extent of secreting surface than the simple membrane; the follicle than the crypt; the cÆcum than the follicle; and the tubulum than the cÆcum. A certain amount of secreting surface is gained by the disposition of the simple membrane into the form of the crypt. The collection of a number of crypts into a cluster doubles the extent of the secreting surface by the extent of every crypt that is added to the cluster. The addition of every cluster doubles the whole extent of surface acquired by a single cluster. But when stems spring as if from a common trunk; when branches spring from a stem; when small branches spring from the large branches, and yet smaller branches from the small in a series, which the eye, assisted by the most powerful microscope, is wholly unable to trace; when all the clusters thus formed are collected, and combined into a compact mass, the intricacy of which no art can completely unravel, the extent of surface obtained is altogether immeasurable. How immense must be the extent of surface thus acquired in such an organ as the human lungs, in such a gland as the human liver!737. In such an aggregation the concentration is also equal to the accumulation; the maximum of surface is comprised in the minimum of space, and the energy and elaborateness of the function of a secreting organ is uniformly proportionate to such a concentration of its secreting substance.

738. Hence the complexity of the compound gland in the higher animals would appear to arise solely from the intricate arrangement of the immense mass of secreting matter concentrated in a small compass; hence also the progressively increased complication indicated in the successive development of the glandular system in the animal series. Thus, for example, among the distinct organs formed for the purpose of elaborating a specific secretion, being intimately connected with the process of digestion, one of the first is the salivary gland. Low down in the scale, in the animal in which the first rudiment of a salivary gland is traceable, it consists of a single follicle, which appears to serve the office of a gland. In an animal a little higher in structure, two, three, or four follicles combine to form a somewhat less simple organ. In an animal still higher in the series, a number of follicles are clustered together and form a much more complex organ; and in this manner, as the organization of the animal becomes higher and higher, the complexity of the gland increases, until at length it is composed of a countless number of follicles collected into clusters, the clusters disposed into lobes, the lobes subdivided into lobules, and the lobules into still smaller particles, the ultimate elements of the glandular apparatus. In like manner, when the first rudiment of the liver is discoverable, it consists of a single pouch or cÆcum; somewhat higher in the series, the organ is composed of two or more cÆca distinct and free; and then, as its complexity increases with the perfection of the organization, cÆca are accumulated upon cÆca; the aggregates so formed are closely compacted, disposed into lobes, divided into lobules, and subdivided into the ultimate particles of the glandular apparatus. So in a gland composed of tubuli, as the kidney, the organ in its rudimentary state consists of a few straight tubuli: as its structure advances more tubuli are added: next, the increasing tubuli superimposed one upon another become tortuous; then the tubuli still accumulating, become not merely tortuous, but convoluted; and last of all, countless numbers of tubuli are closely compacted into exceedingly convoluted masses. Uniformly, the lower the animal and the simpler the organ, the larger and the more manifest are the elementary parts of the gland; but in the higher animals these elementary bodies are so minute as to be altogether microscopical and their arrangement is so complex that it can be unravelled only with extreme difficulty.

739. It is a striking confirmation of the correctness of this view of the structure of the glandular apparatus, that whenever in the ascending series a gland appears for the first time in any class, the elementary bodies are so large, and are disposed in so simple a mode, that a slight examination is sufficient to demonstrate their primitive form, and to render it manifest that they consist either of vesicles, follicles, cÆca, or tubuli, more or less aggregated. This is seen in the obvious structure presented by the liver, the pancreas, the salivary glands, and the mammÆ, in the simple animals in which these organs first appear. Thus the liver in animals low down in the scale is manifestly composed of simple clustering follicles: in the fish the pancreas is composed of simple branched follicles: in the bird, the salivary glands are composed of simple parallel tubuli; and in the cetacea the breasts are composed of simple branched tubuli.

740. But the microscope, by bringing the successive development of the compound gland in the embryo of the higher animal under the cognizance of sense, perfectly discloses the nature of its composition. In the development of the incubated egg every step of the progressive formation of the compound gland is rendered visible to the eye. When this process is carefully watched, it is seen that the part of the gland first formed is the excretory duct, which springs from the blastema, the common mass of matter out of which all the organs are formed. From this duct the elementary parts of the gland bud just as bunches of grapes bud from the stalk. The buds, at first at considerable distances from each other, approach nearer as they increase by new growths, until at length they come into actual contact. The growth continuing, and the compactness of the substance of the gland proportionally increasing, the primitive form of the elementary bodies which compose it is ultimately lost. The substance of the gland now appears to consist of compact solid matter, which is commonly termed parenchyma. The component particles of this parenchymatous and apparently solid substance present a clustered or grape-like appearance, from which they early obtained the name of acini, from the Latin word acinus, a berry. This term, originally employed merely to express the clustered and branching appearance of the elementary parts of the gland, has since been used in widely different senses. By some it has been employed to express solid glandular grains constituting a supposed distinct parenchymatous substance, differing in every different gland. It is now proved that no such solid granular particles enter into the composition of any gland in the animal kingdom. By others the term acini has been employed to express granular bodies composed of blood-vessels, directly continuous with the excretory ducts, and from which the excretory ducts derive their origin. Recent investigation has demonstrated that there is no continuity of the blood-vessels into the excretory duct either in the acini or in any other part of the gland. It is established that the blood-vessels are spread out upon the walls of the secreting canals and do not form with them continuous tubes. The bodies which have been mistaken for granular particles, constituting the so called solid acini, are really the shut extremities of hollow follicles, cÆca, or tubuli, which appear solid only from the closeness with which they are compacted. When carefully dissected and examined under the microscope, their real nature becomes apparent, and this is also sometimes capable of being demonstrated by injection; for some of these elementary bodies are vesicular, and can be filled with mercury, when they present a beautiful appearance like clusters of diamonds; or they may be inflated with air, just as the air vesicles of the lungs.

741. On watching the formation of the gland in the development of the embryo, it would appear that at first free streams of blood, or blood not contained in proper vessels, pass around the acini, the shut extremities of the excretory ducts, or the secreting canals. “So it would seem,” says MÜller, “when we examine the evolution of the liver and kidney in the embryo of the lower animal; for the interstices of the canals appear bloody, without the slightest trace of the walls of blood-vessels. I conceive that in the beginning new streams arise in an amorphous mass (a mass without form), not bounded by proper parieties; but that soon walls are formed, which present definite boundaries to the streams, the density of the substance around the streams gradually increasing.” It is in this manner that the connexion is first established between the system of capillary blood-vessels and that of the secreting organs.742. In its embryo state the compound gland of the highest animal consists of mere excretory ducts, wonderfully similar to the simple secreting bodies of the lowest classes. But in the higher animal this simple form of the gland is transient: gradually, with the progressive evolution of the embryo, it passes into a more complex structure; while in the lower animal the simple form of the gland remains permanently the same through the whole term of life.743. Such are the main points which have been ascertained relative to the structure of the secreting apparatus, which enters in one or other of its forms, as a constituent element, into almost every part of the animal body. Wherever there is nutrition there is secretion, and wherever there is secretion there is one or other of these secreting bodies. How immense the number of these organs in the human body! Every point in the interior of the walls that bound the great cavities is a secreting surface. Every point of the secreting surface that lines the alimentary canal, from its commencement to its termination, is studded with distinct secreting organs. Every point of the skin is still more thickly studded with distinct secreting organs. By the naked eye, and still more distinctly with a lens, may be seen the pores through which the vapour that constitutes the insensible perspiration incessantly exudes. Next are the open mouths of myriads of sebacious follicles that pour out upon the skin the oily matter which gives it its suppleness and softness; and besides all these, are the hairs, each the product of a secreting organ placed immediately beneath the skin. An attempt to count the number of pores and hairs visible to the eye within the compass of an inch, and thence to compute the number on the whole surface of the skin, may convey some conception of the amount of these organs; yet these form but a small part of the secreting apparatus. The great viscera of the body, the brain, the lungs, the liver, the pancreas, the spleen, are portions of it; all the organs of the senses, the eyes, the ears, the nose, the tongue; all the organs of locomotion; every point of the surface of every muscle, and a great part of the surface and substance of the very bones are crowded with secreting organs.744. Since every secreting organ is copiously supplied with blood, it follows that a great part of the blood of the body is always circulating in secreting organs; and, indeed, it is to afford materials for the action of these organs that the blood itself is formed.745. How do these organs act upon the blood? All that is known of the course of that portion of the blood which flows through an organ of secretion is, that it passes into arteries of extreme minuteness, which are spread out upon the external walls of the elementary secreting bodies, and which, as far as they can be traced, pass into capillary veins,—nowhere terminating by open mouths—nowhere presenting visible outlets or pores; their contents probably transuding through their thin and tender coats by the process of endosmose.746. As it is flowing through these capillary arteries, the blood undergoes the transformations effected by secretion, forming—1. The fluids, which are added to the aliment, and which accomplish its solution, and change it into chyme. 2. The fluids, which are added to the chyme to convert it into chyle, and both to chyle and lymph, to assist in their assimilation. 3. The fluids which, poured into the cavities, facilitate automatic or voluntary movements. 4. The fluids, which serve as the media to the organs of the senses by which external objects are conveyed to the sentient extremities of the nerves for their excitement. 5. The fluids which, deposited at different points of the cellular tissue, when more aliment is received than is needed, serve as reservoirs of nutriment to be absorbed when more aliment is required than can be afforded by the digestive organs. 6. The fluids which are subsequently to be converted into solids. 7. The fluids which are eliminated from the common mass, whether of fluids or solids, to be carried out of the system as excrementitious substances. 8. In addition to all these substances, which are indispensable to the preservation of the individual, those which are necessary to the perpetuation of the species.747. In order to form any conception of the mode in which the secreting organs act upon the blood, so as to elaborate from it such diversified substances, it is necessary to consider the chemical composition of the different products of secretion, and the degrees in which they really differ from each other, and form the common mass of blood out of which they are eliminated.748. By chemical analysis, it is established that all the substances which are formed from the blood by the process of secretion are either water, albumen, mucus, jelly, fibrin, oil, resin, or salts; and, consequently, that all the secretions are either aqueous, albuminous, mucous, gelatinous, fibrinous, resinous, oleaginous, or saline.749. 1. Aqueous Secretions.—From the entire surface of the skin, and also from that of the lungs, there is constantly poured a quantity of water, derived from the blood, mixed with some animal matters, which, however, are so minute in quantity, that they do not communicate to the aqueous fluid any specific character.750. 2. Albuminous Secretions.—All the close cavities, as the thorax, the abdomen, the pericardium, the ventricles of the brain, and even the interstices of the cellular tissue, are constantly moistened by a fluid which is termed serous, because it is derived from the serum of the blood. This serous fluid consists of albumen in a fluid form, and it differs from the serum of the blood chiefly in containing in equal volumes a smaller proportion of albumen. Membranes of all kinds consist essentially of coagulated albumen; and the albumen, as constituting these tissues, differs from albumen as existing in the serum of the blood only in being unmixed with extraneous matter, and in being in a solid form.751. 3. Mucous Secretions.—As all the close cavities, or those which are protected from the external air, are moistened with a serous fluid, so all the surfaces which are exposed to the external air, as the mouth, the nostrils, the air-passages, and the whole extent of the alimentary canal, are moistened with a mucous fluid. Mucus does not exist already formed in the blood. It is always the product of a gland. Some of the mucous glands are among the most elaborate of the body; still the main action of the gland seems to be to coagulate the albumen of the blood, for the basis of mucous is coagulated albumen. The fluid that lubricates the mucous surfaces in their whole extent, the saliva, the gastric juice, the tears, the essential part of the fluid formed in the testes and in the ovaria, are mucous secretions. Hence the most complex and elaborate functions of the body, respiration, digestion, reproduction, are intimately connected with the mucous secretions: nevertheless, as far as regards their chemical nature, the mucous differ but slightly from the albuminous secretions; and it is probable that a slight change in the secreting organ is sufficient to convert the one into the other. By the irritation of mercury on the salivary glands, the saliva, properly of a mucous, is sometimes converted into a substance of an albuminous nature; and irritation in some of the serous membranes occasionally causes them to secrete a mucous fluid.752. 4. Gelatinous Secretions.—The proximate principle termed jelly abounds plentifully in several of the solids of the body, and more especially in the skin; but jelly does not exist already formed in the blood. Yet it is not the product of a gland, neither is there any known organ by which it is formed. Out of the body albumen is capable of being converted into jelly by digestion in dilute nitric acid: this conversion is probably effected by the addition of a portion of oxygen to the albumen. Albumen contains more carbon and less oxygen than jelly; the proportions of hydrogen and nitrogen in both being nearly the same. According to MM. Gay Lussac and ThÉnard, the elements of albumen and jelly are,

The conversion of albumen into jelly is incessantly going on in the system; and the process accomplishes most extended and important uses. In the lungs at the moment of inspiration oxygen enters into the blood in a state of loose combination; but in the system, at every point where the conversion of albumen into jelly takes place, oxygen probably enters into a state of chemical combination with albumen; and the new proximate principle, jelly, is the result. The agent by which this conversion is effected appears to be the capillary artery: the primary object of the action is the production of a material necessary for the formation of the tissues of which jelly constitutes the basis, as the skin; but a secondary and most important object is the production of animal heat; the carbon that furnishes one material of the fire being given off by the albumen at the moment of its transition into jelly; and the oxygen that furnishes the other material of the fire being afforded to the blood at the moment of inspiration. This view affords a beautiful exposition of the reason why jelly forms so large a constituent of the skin in all animals. The great combustion of oxygen and carbon, the main fire that supports the temperature of the body, is placed where it is most needed, at the external surface.753. 5. Fibrinous Secretions.—The pure muscular fibre, or the basis of the flesh, is identical with the fibrin of the blood. It contains a larger proportion of nitrogen, the peculiar animal principle, and is consequently more highly animalized than the preceding substances. It appears to be simply discharged from the circulating blood by the capillary arteries, and deposited in its appropriate situation; no material change in its constitution being, it would seem, necessary to fit it for its office.754. 6. Oleagenous Secretions.—Fat of all kinds, which is found so extensively connected with the muscles, and with many of the viscera, and which is more or less diffused through the whole extent of the cellular tissue, marrow, milk, and nervous and cerebral matter, are essentially of the same nature. The basis of them all is oil; and oil exists already formed both in the chyle and in the blood.755. 7. Resinous Secretions.—The peculiar substance forming the basis of bile, picromel; the peculiar substance forming the basis of urine, urea; the peculiar substance connected with the muscular fibre, and forming a component part of almost all the solids and fluids of the body, osmazome, consists of a common principle—a resin, which exists already formed in the blood, and more especially in the serosity of the blood.756. 8. Saline Secretions.—The substances termed saline, namely, the acids, the alkalis, and the neutral and earthy salts, are disposed over every part of the system: they enter more or less into all the constituents both of the solids and fluids; they form more especially the phosphate of lime, the earthy matter of which bones are composed; and they all exist already formed in the blood.757. From this account, then, it appears, that by chemical analysis, the blood is ascertained to contain water, albumen, fibrin, oil, resin, and various saline and earthy substances: it follows, that, with the exception of the absence of jelly, the constituents of the body and the constituents of the blood are nearly identical; and it is probable that they will be found to be perfectly identical when their analysis shall have become complete.758. It is also manifest that in by far the greater number of cases the various substances of which the body is composed are simply separated from the nutritive fluid at the parts of the body at which they are deposited; and that, existing already formed in the blood, they are merely deposited there, and not generated. Still, however, since it is certain that gelatin cannot be recognized in the blood, and since it is doubtful whether some other substances found in different textures and secretions really exist in the blood, it is necessary, in the present state of our knowledge, to suppose, that although most of the constituents of the living tissues are contained in the blood, yet that in some instances a material change is effected in their nature at the time and place of their escape from the circulation; and that in these cases the secreted substances are not simple extracts from, but products of, the blood.759. It is by the apparatus of secretion that this separation, evolution, or re-formation, is effected. Out of a fluid which contains, blended together, almost all the heterogeneous substances of which the body is built up, particular substances are selected from the common mass, and are deposited in certain parts, and only in certain parts. Although by the most careful examination of the structure of the apparatus, it is not possible to form a precise conception of the mode in which this separation is effected, yet we are enabled to perceive a number of contrivances which we can readily understand must conduce to the accomplishment of the object.760. 1. Of these, the most obvious is mechanical arrangement.761. In its passage to different organs the blood is propelled through canals of extreme minuteness: in every different case these canals differ from each other in size; pass off from their respective trunks at different angles; possess different degrees of density; are variously contorted, and are of various lengths. In some they are straight, in others convoluted; at one time branching, at another pencillated, and at another starry. The veins, too, in some cases, are almost straight, in others exceedingly tortuous, in others reticulated; and the freedom of their communication with the arteries varies so much, that in some cases fine injections pass from the one set of vessels to the other with the greatest facility, while, in others they pass with extreme difficulty. The consequence of these divers arrangements of the capillary blood-vessels is, that the current of the blood must necessarily flow in them with different degrees of velocity; its particles must be placed at different distances from each other, and must be presented to each other in different positions and in widely different proportions. In no two secreting organs are any two of these conditions exactly alike. In the lower orders of animals, in which secretion is seen in its simplest condition, the general nutritive fluid, elaborated and contained in a single internal cavity, appears to furnish a variety of products very different from itself, by a process hardly more complex than mere transudation through a living membrane. In the higher animals the different secreting organs may be considered, in part at least, as mechanical contrivances adapted to carry on analogous transudations—fine sieves or strainers diversly constructed. A fluid containing such heterogeneous matters as the blood, held in combination by so slight an affinity, slowly transuding through series of tubes, the mechanical arrangement of which is so varied, must yield a different substance in every different case. Thus by simply filtering the blood a vast variety of products may be obtained, merely in consequence of a varied disposition of the minute tubes of which the filters are composed.762. 2. But in the second place, this diversity of mechanical arrangement is calculated in a high degree to promote and to modify chemical action. The contact or proximity of the particles of bodies, the extent of surface which those particles present to each other, the space of time in which they continue in contact, the degree of force with which they impinge against each other, the degree of temperature to which they are exposed,—these, and circumstances such as these, are conditions which exert the most powerful influence over chemical decomposition and re-combination. In the different secreting organs, as has been shown, the blood must necessarily pass through vessels having every conceivable diversity of diameter: in those vessels it must consequently flow with corresponding differences of velocity. Some of these diameters will admit one constituent of the blood, as one of the red particles; others may be large enough to admit two or more of the red particles abreast; others may be so small as to be incapable of admitting a single red particle, receiving only the more fluid portions of the blood; in some vessels these different constituents will be in one degree of proximity, in others in another; in some they will remain long in contact, in others only for an instant: it is obvious that from such different conditions the chemical products may be infinitely varied.763. Such is the composition of chemical bodies, that a great diversity of substances is obtainable merely by changing one condition, the proportions in which the elementary particles combine.764. Oxygen and nitrogen combined in one proportion form atmospheric air; in another proportion, nitrous oxide; in another, nitric oxide; in a fourth, nitrous acid; and in a fifth, nitric acid. Few secretions formed from the blood differ more widely from each other than the products thus formed from these two elementary bodies.765. Urea consists of two prime equivalents of hydrogen, one of carbon, one of oxygen, and one of nitrogen. Remove one of the atoms of hydrogen, and take away the atom of nitrogen, urea is converted into sugar; combine with urea an additional atom of carbon, it is changed into lithic acid. In like manner add a small quantity of water to farina, it is converted into sugar; to fibrin, it is changed into adipocere. From a reservoir containing a quantity of substances in the state of vinous fermentation, draw off portions of the liquor at different stages of the process, and cause these to pass through tubes of various diameters and with various degrees of velocity, there will be obtained at one time an unfermented syrup, at another, a fermenting fluid, at another, wine, at another, vinegar. Out of the body place the blood in a state of rest, it will spontaneously separate into serum and crassamentum, and the crassamentum will further separate into fibrin and red particles. Add to the serum a certain portion of acid, it will be coagulated into solid albumen; add to this solid albumen another portion of acid, it will be converted into jelly. Add a certain portion of acid to fibrin, it will be changed into adipose matter; bring the acid into contact with the red particles, they will be converted into a substance closely resembling bile. If by the rough chemistry which the art of man can conduct so great a variety of substances may be obtained out of a single compound, is it not wonderful that a far greater variety should be produced by the delicate and subtle chemistry of life.766. 3. But a third most important agent in the process of secretion is some influence derived from the nervous system.

1. It is proved, by direct experiment, that the destruction of the nervous apparatus, or of any considerable portion of it, stops the process of secretion. By experiments performed by Mr. Brodie, it is ascertained that the secretion of the urine is suspended by the removal or destruction of the brain, though the circulation be maintained in its full vigour by artificial respiration.

2. The section, and still more the removal, of a portion of the sentient nerves of the stomach (the par vagum, or eighth pair), according to some experimentalists, deranges and impedes; according to others, totally arrests the process of digestion.

3. Other classes of phenomena illustrate in a striking manner the influence of the nervous system over the process of secretion. The sight, nay, even the thought of agreeable food, increases the secretions of the mouth. Pleasurable ideas excite, painful ideas destroy, the appetite for food; probably, in the one case, by increasing, and, in the other, by suspending the secretion of the gastric juice: the emotion of grief instantly causes a flow of tears; that of fear, of urine; the sight or thought of her child fills the maternal breasts with milk, while the removal of the child from the mother diminishes and ultimately stops the secretion.767. Even the imagination is capable of exerting a powerful influence over the process. A female who had a great aversion to calomel was taking that medicine in very small doses for some disease under which she was labouring. Some one told her that she was taking mercury: immediately she began to complain of soreness in the mouth; salivated profusely, and even put on the expression of countenance peculiar to a salivating person. On being persuaded that she had been misinformed, the discharge instantly began to diminish, and ceased altogether in a single night. Two days afterwards she was again told, on good authority, that calomel was contained in her medicines, upon which the salivation immediately began again, and was profuse. That this salivation was not produced by the calomel, but was the effect solely of the influence of imagination on the salivary glands, was proved by the absence of redness of the gums, which always takes place in mercurial salivation, and also by the absence of the peculiar fÆtor, which is characteristic of the action of this metal on the system.768. The same influence is apparent even in the lower animals: exhibit food to a hungry dog, the saliva will pour from its mouth. Rob the nest of the bird of its eggs as soon as they are laid, the bird may be made to deposit eggs almost without end, though if the eggs are allowed to remain undisturbed, it will lay only a certain number. The bird is led by instinct to continue to deposit eggs in the nest until a certain number is accumulated; that is, a mental operation acts upon the ovarium, the secreting organ in which the eggs are formed, maintaining it in a state of active secretion for an indefinite period; whereas without that mental operation the secretion would be limited to a definite number.769. In all these cases it is probable that the vital agent by which the effect is produced on the secreting organs is the organic nerve. Though the sentient part of the nervous system may in many cases be the part primarily acted on, yet there is reason to believe that the ultimate effect is invariably produced on the organic part, the sentient nerves in this case acting on the organic, as in other cases the organic act on the sentient, in consequence of that intimate connexion which, for the reason assigned (vol. i. p. 79), is established between both parts of this system. For,770. 1. The true object of the sentient part of the nervous system is to establish a relation between the body and the external world; the object of the organic part is to preside over the functions by which the body is sustained and nourished, that is, over the processes of secretion.771. 2. The nerves which are distributed to the secreting arteries, and which increase in number and size as the arteries become capillary, are, for the most part, derived from the organic portion of the nervous system (fig. CLXX. 3). This anatomical arrangement clearly points to some physiological purpose, and indicates the closeness of the relation between the function of the organic nerve and the ultimate action of the capillary artery.772. 3. It is demonstrated that the sentient part of the nervous system, though occasionally influencing and modifying secretion, is not indispensable to it. In tracing the normal or regular development of the human foetus, it is found that the heart is constructed and is in full action before the brain and spinal cord, the central masses of the sentient part of the nervous system, are in existence; and that these masses are themselves built up by processes to which the action of the heart is indispensable; consequently, innumerable acts of secretion must have taken place, those, for example, which have been necessary to form the different substances which enter into the composition of the heart, before the brain and spinal cord exist. In like manner in the anormal or irregular development of the foetus, as in the production of monsters, there may be not a vestige of head, neck, brain or spinal cord, while there may be a perfect heart, perfect lungs, perfect intestines, and various portions even of the osseous system.773. However in the perfect animal secretion may be under the influence of the brain and spinal cord, it is clear that, since the process can go on without them, it must be independent of them. It is a false induction from these facts drawn by some physiologists that secretion is independent of the nervous system. They do prove that it is independent of one part of the nervous system, the sentient; but it does not follow that it is independent of the other part, the organic.774. 4. It is demonstrated that the organic part of the nervous system is not only independent of the sentient part, but that it is even pre-existent to it. Researches into the development of the nervous system, as shown in the progressive growth of the foetus of different animals, have proved that the existence of the organic nerves is manifest long before that of the sentient; that nerves are discoverable in the tissues, before the brain and the spinal cord are formed; that as these masses become visible and grow, nerves springing from the tissues advance towards the central nervous masses, and at length unite with them; but that this union does not take place until the development of the nervous system is considerably advanced. These curious and most instructive facts show that in the foetus, though the brain and spinal cord may have been destroyed or have been non-existent, yet that the organic nerves may have been in full action. After a communication has been once established between the two parts of the system, indeed, the destruction of the brain or spinal cord may stop secretion, not because these organs are indispensable to secretion; but because the destruction of one part of the system involves the death of the other, just as the organic life itself perishes soon after the destruction of the animal.775. The existence of the organic nerve is probably simultaneous with that of the secreting artery: from the first to the last moment of life the nerve regulates the artery; the influence of the one is indispensable to the operation of the other; and, by their conjoint action, the sentient nerve itself, as well as every other organ, is constructed.776. There is reason to believe that the physical agent by which the organic nerve influences secretion is electricity. The nerve appears to be the medium by which electrical fluid is conveyed to the secreting organs, and the nerve probably influences secretion by influencing chemical combination, through the intervention of this most powerful chemical agent. This is rendered probable by the observation of various phenomena, and by the result of direct experiment.777. 1. It is proved that galvanic phenomena may be excited by the contact of the nerve and muscle in an animal recently dead. A galvanic pile may be constructed of alternate layers of nervous and muscular substance, or of nervous substance and other animal tissues. A secreting organ liberally supplied with organic nerve is probably then in its physical structure nothing but a galvanic apparatus. It is certain that some animals, as the raia torpedo, possess a special electrical apparatus composed essentially of nervous matter; that the nerves which compose this apparatus correspond strictly with the organic nerves of the human body; that they are distributed principally to the organs of digestion and secretion, and that they exert a powerful influence over these processes; for, when the animal is frequently excited to give shocks, digestion appears to be completely arrested; so that, after the animal’s death, food swallowed some time previously is found wholly unchanged.778. 2. It is universally admitted that the nerves in all animals possess an extreme sensibility to the stimulus of electricity, and more especially to that form of it which is termed galvanism.779. 3. Direct experiment proves that the stimulus of galvanism may be made to produce in the living-body precisely the same effect as the nervous influence. It has been stated, that the division of the par vagum, in the neck of a living animal, suspends the digestion of the food probably by stopping indirectly the secretion of the gastric juice. If after the division of the nerves, their lower ends, that is, that portion of the nerves which is still in communication with the stomach, but no longer in communication with the brain, be made to conduct galvanic fluid to the stomach, secretion goes on as fast as when the nerves are entire and conduct nervous influence. Dr. Wilson Philip having divided the par vagum in the neck of a living animal, coated a portion of the lower end of the nerves with tin foil, placed a silver plate over the stomach of the animal, and connected respectively the tin and silver with the opposite extremities of a galvanic apparatus. The result was that the animal remained entirely free from the distressing symptoms which had always before attended the division of the nerves, and that the process of digestion, which had been invariably suspended by this operation, now went on just as in the natural state of the stomach. On examining the stomach after death, the food was found perfectly digested, and afforded a striking contrast to the state of the food contained in the stomach of a similar animal, in whom the nerves had been divided, but which had not been subjected to the galvanic influence.780. 4. On applying a low galvanic power to a saline solution contained in an organic membrane, Dr. Wollaston found that the galvanic fluid decomposed the saline solution, and that the component parts of the solution transuded through the membrane; each constituent being separately attracted to the corresponding wire of the interrupted circuit. This experiment, says this acute and philosophical physiologist, illustrates in a very striking manner the agency of galvanism on the animal fluids. Thus the quality of the secreted fluid may probably enable us to judge of the electrical state of the organ which produces it; as for example, the general redundance of acid in urine, though secreted from blood that is known to be alkaline, appears to indicate in the kidney a state of positive electricity; and since the proportion of alkali in bile seems to be greater than is contained in the blood of the same animal, it is not improbable that the secretory vessels in the liver may be comparatively negative.781. We may imagine, says Dr. Young, that at the division of a minute artery a nervous filament pierces it on one side, and affords a pole positively electrical, and another opposite filament a negative pole. Then the particles of oxygen and nitrogen contained in the blood, being most attracted by the positive point, tend towards the branch which is nearest to it; while those of the hydrogen and carbon take the opposite channel; and that both these portions may be again subdivided, if it be required; and the fluid thus analysed may be recombined into new forms by the reunion of a certain number of each of the kinds of minute ramifications. In some cases the apparatus may be somewhat more simple than this; in others, perhaps, much more complicated; but we cannot expect to trace the processes of Nature through every particular step; we can only inquire into the general direction of the path she follows.782. Considerations such as these afford us a glimpse into the mode in which Nature conducts some of her most secret and subtile operations; or rather into the immediate agency by which she effects them; for, properly speaking, of the mode in which she works, we do not obtain the slightest insight, and even of her immediate agency our view, at least in the present state of our knowledge, is indistinct and vague. By the study of the apparatus which she builds up, we can trace back her operations a step or two; but in every case, at a certain point, the apparatus itself becomes so delicate as to elude our senses, and then of course we are necessarily at a stand. So, the rough materials with which she carries on her great work of secretion, by careful analysis we can separate into divers parts, and ascertain that each part possesses peculiar properties. The main channels by which she conveys these varied constituents to the different parts of the system we can trace; the delicate organs by which she produces on these rude materials her wonderful transformations we can see; but beyond the threshold of these organs we cannot go. Why from one common mass of fluid the same variety of peculiar substances are constantly separated, and each in its respective place: why the kidney never secretes milk, nor the liver urine, nor the breast bile: why membrane, and muscle, and bone, and fat, and brain, are uniformly deposited in the same precise situation: why these depositions go on with uniformity, constancy and regularity; and by what laws each process is controlled and modified, we do not know. But though with whatever diligence we investigate these operations, the great problem remains, and probably ever will remain unresolved, still it is both a pleasurable and a profitable labour to follow Nature in her path, to the extreme point to which it is possible to trace her footstep; for the phenomena themselves are often in the highest degree curious and interesting; while their order and relation can seldom be so considered as to be understood, without the suggestion of practical applications of great and permanent usefulness.