SECRETION.

We have yet to describe an important and multiple function, which takes place in the very tissue of our organs—in the capillary system, and which separates from the blood the various humours of the body. This is the function of secretion,—a term which has been applied both to the operation and the product. Thus, the liver is said to separate the bile from the blood by an action of secretion, and the bile is said to be a secretion.

The organs that execute the various secretory operations, differ greatly from each other. They have, however, been grouped by anatomists in three classes, each of which will require a general notice.

Anatomy of the Secretory Apparatus.

The secretory organs have been divided into exhalant, follicular, and glandular.

The remarks made respecting the exhalant vessels, under the head of nutrition, will render it unnecessary to allude, in this place, to any of the apochryphal descriptions of them, especially as their very existence is supposititious. Many, indeed, imagine them to be nothing more than the minute radicles of ordinary arteries.

The follicle or crypt has the form of an ampulla or vesicle, and is situated in the substance of the skin and mucous membranes; secreting a fluid for the purpose of lubricating those parts. In the exhalant vessel, the secreted fluid passes immediately from the blood-vessel, without being received into any excretory duct; and in the follicle there is essentially no duct specially destined for the excretion of the humour.

The follicle is membranous and vascular, having an internal cavity in which the secretion is accomplished; and the product is poured upon the surface, beneath which it is situated, either by a central aperture, or by a very short duct—if duct it can be called— termed a lacuna.

The gland is of a more complex structure than the last. It consists of an artery, which conveys blood to it; of an intermediate body, the gland, properly so called,—and of an excretory duct to carry off the secreted fluid, and pour it on the surface of the skin or mucous membranes. The blood-vessel, which conveys to the gland the material from which the secretion has to be operated, enters the organ at times by various branches; at others, by a

single trunk, and ramifies in the tissue of the gland; communicating at its extremities with the origins of the veins and of the excretory ducts. These ducts arise by fine radicles at the part where the arterial ramifications terminate; and they unite to form larger and less numerous canals, until they terminate in one large duct, as in the pancreas; or in several, as in the lachrymal gland; the duct generally leaving the gland at the part where the blood-vessel enters. Of this we have a good exemplification in the kidney,

(see Fig. 123.)

Besides these vessels, veins exist, which communicate with the vessels that convey blood to the gland, both for the formation of the humour and the nutrition of the organ, and return the residuary blood to the heart. Lymphatic vessels likewise exist; and nerves, which proceed from the ganglionic system, form a network around the secreting arteries, as in Fig. 120, accompany them into the interior of the organ, and terminate, like them, invisibly.

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Fig. 120.

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a a. A portion of intestine.-bb. Part of the aorta.-c c. Nerves following the branches of the aorta, to supply the intestine.

BORDEU was of opinion, that the glands, judging from the parotid, are largely supplied with nerves. The nerves, however, do not all belong to it, merely crossing it in their course to other parts. Bichat, from the small number sent to the liver, was induced to draw opposite conclusions to those of Bordeu.

These may be looked upon as the great components of the glandular structure, along with cellular membrane, to bind them together, and, at times, an outer envelope.

The intimate texture of these organs has been a topic of much speculation. It is generally considered, that the final ramifications of the arterial vessels, with the radicles of the veins and excretory ducts, and the final ramifications of the lymphatic vessels and nerves, form so many small lobules, composed of minute, granular masses. Such, indeed, is the appearance the texture presents, when examined by the naked eye. Each lobule is conceived to contain a final ramification of the vessel or vessels conveying blood to the organ, a nerve, a vein, a lymphatic, and an excretory duct,—with cellular tissue binding them together. When the organ has an external membrane, it usually forms a sheath to the various vessels. The lobated structure is not, however, equally apparent in all the glands. It is well seen in the pancreas, and in the salivary and lachrymal.

The precise mode in which the blood-vessel, from the blood of which the secretion is effected, communicates with the excretory duct, does not admit of detection. Some have supposed, that between the termination of the blood-vessel and the commencement of the duct, a secretory vessel, specially charged with the function, exists, which conveys the secreted humour into the excretory duct. Of this, however, we have no evidence; and the existence of any, except the minute capillary vessel, appears needless.

Malpighi maintained, that such glands as the liver are composed of very minute bodies, called acini, from their resemblance to the stones of grapes; that these acini are hollow internally, and are covered externally by a net-work of blood-vessels; and that these minute blood-vessels pour into the cavities of the acini the secreted fluid, from which it is subsequently taken up by the excretory ducts.

Ruysch, however, satisfied himself, that the acini of Malpighi are merely convoluted vessels, and that they are continuous with the excretory ducts. In Malpighi's view, the secretory organ is a mere collection of follicles: in Ruysch's, simply an exhalant membrane variously convoluted. The latter is the view almost universally embraced by the anatomist and physiologist. "The chief, if not the only difference," says Sir Charles Bell, "between the secreting structure of glands and that of simple surfaces, appears to consist in the different number and the different arrangement of their capillary vessels. The actual secreting organ is in both cases the same,—capillary blood-vessel; and it is uncertain whether either its peculiar arrangement, or greater extent in glandular texture, be productive of any other effect than that of furnishing the largest quantity of blood-vessels within the smallest space. Thus convoluted and packed up, secreting organ may be procured to any amount that may be required, without the inconveniences of bulk and weight."

It is manifest, then, that the three classes of secretory organs,

however different they may appear to be, are essentially varieties of the same structure; that the capillary vessel is the important agent of the secretion; that the simplest form of the secretory apparatus is this simple capillary vessel; and that the follicles and glands are structures of a more complex organization.

Physiology of Secretion.

The uncertainty which rests upon the intimate structure of secreting organs, and upon the mode in which the different bloodvessels communicate with the commencement of the excretory duct, envelopes the function, executed by those parts, in obscurity. We see the pancreatic artery pass to the pancreas, ramify in its tissue, become capillary, and escape detection; and we see other vessels becoming larger and larger, and emptying themselves into vessels of greater magnitude, until, ultimately, all the secreted humour is contained in one large duct, which passes onwards and discharges its fluid into the small intestine. Yet if we follow the pancreatic artery as far as the eye can carry us, even when aided by glasses of considerable magnifying power, or if we trace back the pancreatic duct as far as is practicable, we find, in the former vessel, always arterial blood, and, in the latter, always pancreatic juice. It must, consequently, be between the part at which the artery ceases to be visible, and the pancreatic duct becomes so, that secretion is effected; and we cut the knot by asserting, that it occurs in the very tissue, parenchyma, or in the capillary system of the secretory organ.

Conjecture, in the absence of positive knowledge, has been busy, at all times, in attempting the explanation of the mysterious agency by which we find such various humours separated from the same fluid; and, according as chymical, or mechanical, or vital doctrines have prevailed in physiology, the function has been referred to one or other of those agencies.

The general belief, amongst the physiologists of the sixteenth and seventeenth centuries, was, that each gland possesses a peculiar kind of fermentation, which assimilates to its own nature the blood passing through it. The notion of fermentation was, indeed, applied to most of the vital phenomena. It is now totally abandoned, owing to its being purely imaginary, and inconsistent with all our ideas of the vital operations. When this notion passed away, and the fashion of accounting for physiological phenomena on mechanical principles usurped its place, the opinion prevailed, that the secretions are operated through the glands as through filters. To admit of this mechanical result, it was maintained, that all the secreted fluids exist ready formed in the blood, and that when they respectively arrive at the different secretory organs, they pass through, and are received by the excretory ducts.

Descartes and Leibnitz were warm supporters of this mecha

nical doctrine, although their views differed materially with regard to the precise nature of the operation. Descartes supposed, that the particles of the various humours are of different shapes, and that the pores of the glands have respectively a corresponding figure; so that each gland permits those particles only to pass through it, which have the shape of its pores. Leibnitz, on the other hand, likened the glands to filters, which had their pores saturated with their own peculiar substance, so that they admitted this substance alone to pass through them, and excluded all others,as paper, saturated with oil, will prevent the filtration of water, and

vice versa.

The mechanical doctrine of secretion was taught by Malpighi and by Boerhaave and continued to prevail even till the time of Haller. All the secretions were conceived to be ready formed in the blood, and the glands were looked upon as sieves or strainers to convey off the appropriate fluids or humours. In this view of the subject, all secretion was a transudation through the coats of the vessels, the blood globules of various sizes passing through pores adapted to them.

The mechanical doctrine of transudation, in this shape, is founded upon supposititious data; and the whole facts and arguments are so manifestly defective, that no refutation is necessary. It is now, indeed, wholly abandoned. MM. Magendie and Fodera have, however, revived the mechanical doctrine of late years, but under an essentially different form; and one applicable especially to the exhalations.

The former gentleman, believing that many of the exhalations exist ready formed in the blood, thinks that the character of the exhaled fluid is dependent upon the physical arrangement of the small vessels, and his views repose upon the following experi

ments.

If, in the dead body, we inject warm water into an artery passing to a serous membrane, as soon as the current is established from the artery to the vein, a multitude of minute drops are observed oozing through the membrane, which speedily evaporate.

If, again, a solution of gelatine, coloured with vermilion, be injected into all the vessels, it will often happen, that the gelatine is deposited around the cerebral convolutions, and in the anfractuosities, without the colouring matter escaping from the vessels, whilst the latter is spread over the external and internal surface of the choroid. If again, linseed oil, also coloured with vermilion, form the matter of the injection, the oil, devoid of colouring matter is deposited in the articulations furnished with large synovial capsules, whilst no transudation takes place at the surface of the brain, or in the interior of the eye.

Magendie asks, if these are not instances of true secretion taking place post mortem, and evidently dependent upon the physical arrangement of the small vessels; and whether it is not ex