CHAPTER IX.

OF SECRETION.

1. Of the secreting process in general; and of the instruments by which it is effected.

710. We have seen that, in the process of Nutrition, the circulating current not only deposits the materials, which are required for the renovation of the solid tissues; but also takes back the substances, which are produced by the decay of these, and which are destined to be thrown off from the body. We have also seen, that it supplies the materials of certain fluids, which are separated from it to effect various purposes in the economy; such as the Salivary and Gastric fluids, which have for their office to assist in the reduction of the food. Now the process by which the fluids of the latter kind are separated from the Blood, is precisely the same in character as that by which the products of decay are eliminated from it; and the structure of the organs concerned in the two is essentially the same. Hence both processes are commonly included under the general term Secretion, which simply denotes separation. Considered in its most general point of view, this designation may be applied to every act, by which substances of any kind are separated from the blood. Thus the function of the cells, which are concerned in the elaboration of the organizable plasma, may be termed one of Secretion, because they draw from the blood a supply of Albumen, upon which their converting action is exercised; but as the product of their operation is returned to the blood again, and is employed for higher purposes in the economy, the process is usually termed Assimilation. In the same manner, the elaborating action of the Lymphatic Glands, with the Spleen, Thymus Gland, &c., is not usually termed Secretion; since, although it is exercised upon matter drawn from the blood, the product appears to be delivered back into the circulating current, through the medium of the Lymphatic System. (CHAP. V.) With much more justice, however, the process of Respiration may be regarded as one of Secretion; for it consists, as we have seen, in the constant elimination of a substance from the blood, which cannot be retained in it without the most injurious consequences.

711. There is an important difference in the characters of the principal products of the Secreting process, which is connected with the purposes that are to be answered by their separation. Some of these products are altogether different from the ordinary constituents of the animal fabric, and from the materials which the blood supplies for the nutrition of these. So different are they, that their presence in the circulating current has an injurious effect; and the great object of their separation is the maintenance of the purity of the blood. These poisons, for such they may be considered, are generated in the system by the decay and decomposition to which its several parts are liable; and they are just as noxious to it, as if they were absorbed from without.

We have seen that the retention of Carbonic acid in the blood for even a few minutes is fatal, both by putting a stop to the circulation, and by acting unfavorably upon the substance of some of the most important organs in the body. The same fatal result attends the retention of Urea and of Biliary matter, which are among the other products of the decomposition of the tissues; but, although as certain, it is not so speedy, because the general circulation is not affected by the loss of secreting power on the part of the Kidneys and the Liver, and because the accumulation of the noxious matter is slower.-On the other hand, the ingredients that are met with in those secreted fluids, which are destined to answer some purpose in the economy, almost invariably bear a close correspondence with the ordinary materials of the blood. Thus in the Salivary, Gastric, Pancreatic, and Lachrymal fluids, the principal part of the solid matter consists of the saline and of the albu minous constituents of the blood, the latter in a more or less altered condition. In Milk, again, we trace the ordinary constituents of the blood, with very little change. Thus it appears, that the separation of these fluids is not required so much to maintain the Blood in a state of purity, as to supply what is needed for some subsequent operation in the economy; and hence, if the secreting process be interrupted, in the case of any one of them, the suspension has usually no further effect, than that of disturbing the process to which the fluid is usually subservient. If the secretion of Gastric fluid be checked, for example, under the influence of strong mental emotion, the Digestive operation is prevented from taking place.

712. The essential character of the true Secreting operation seems to consist, not in the nature of the action itself, for this is identical. with those of Assimilation and Nutrition, being, as we have seen (§ 239), a process of cell-growth,-but in the position in which the cells are developed, and the mode in which the products of their action are afterwards disposed of. Thus the cells at the extremities of the intestinal Villi (§ 243), the cells of which the Adipose tissue is made up (§ 257), and the cells of which the greater part of the substance of the Liver is formed (§ 239), all have an attraction for fatty matter; and draw it from the neighboring fluids, at the expense of which they are developed, to store it up in their own cavities. But the cells of the first kind, when they have come to maturity, set free their contents, which are delivered to the absorbent vessels, to be introduced into the circulating current ;-those of the second kind seem more permanent in their character, and retain their contents, so as to form part of the ordinary tissues of the body, until they are required to give them up for other purposes, when the matters which they have temporarily separated from the circulating current, are restored to it again without change; and the cells of the third class, when they liberate their contents (which they are continually doing), cast them forth into the hepatic ducts, by which they are carried into the intestinal canal, whence a portion of them at least is directly conveyed out of the body.

713. It is, then, in the position of the Secreting cells,-which causes the product of their action to be delivered upon a free surface, communicating, more or less directly, with an external outlet, that their

distinctive character depends. All the proper Secretions are thus either poured out upon the exterior of the body, or into cavities provided with orifices that lead to it. Thus we shall see that a considerable quantity of solid matter, and a very large quantity of fluid, of which it is desirable that the system should be freed, are carried off from the Cutaneous surface. Another most important secretion, containing a large quantity of solid matter, and serving also to regulate the quantity of fluid in the body,-namely, the Urinary,-is set free by a channel expressly adapted to convey it directly out of the body. The same may be said of the Mammary Secretion, which is separated from the blood, not to preserve its purity nor to answer any purpose in the economy of the individual, but to afford nutriment to another being. And of the matters secreted by the very numerous glandulæ situated in the walls of the Intestinal canal, a great part are obviously poured into it for no other purpose, than that they may be carried out of the body by the readiest channel.

714. The cells covering the simple membranes that form the free surfaces of the body, whether external or internal, are all entitled to be regarded as secreting cells, since they separate various products from the blood, which are not again to be returned to it. But the secreting action of some of these seems to have for its object the protection of the surface; thus the Epidermic cells secrete a horny matter, by which density and firmness are imparted with the cuticle; whilst by the epithelial cells of the Mucous membrane of the alimentary canal, and of other parts, their protective Mucus seems to be elaborated. But in general we find that special organs, termed Glands, are set apart for the production of the chief secretions; and we have now to consider the essential structure of these organs, and the mode of their operation. A true Gland may be said to consist of a closely-packed collection of follicles, all of which open into a common channel, by which the product of the glandular action is collected and delivered. The follicles contain the secreting cells in their cavities; whilst their exterior is in contact with a network of blood-vessels, from which the cells draw the materials of their growth and development (Fig. 94). In any one of the higher animals, we may trace out a series of progressive stages of complexity, in the various glands included within their fabric; and in following any one of the glands that attain the highest degree of development (such as the Liver or Kidney), through the ascending scale of the Animal series, we should trace a very similar gradation from the simplest to the most complex form.

715. That there is nothing in the form or disposition of the components of the glandular structure, which can have any influence upon the character of the secretion it elaborates, is evident from the fact, that the very same product,-e. g., the Bile, or the Urine,-is found to issue from nearly every variety of secreting structure, as we trace it through the different groups of the Animal kingdom. The peculiar power by which one organ separates from the blood the elements of the Bile, and another the elements of the Urine, whilst a third merely seems to draw off a certain amount of its albuminous and saline constituents, is obviously the attribute of the ultimate secreting cells,

which are the real agents in the secreting process (§ 239). Why one set of cells should secrete Bile, another Urea, and so on, we do not know; but we are equally ignorant of the reason, for which one set of cells converts itself into Bone, another into Muscle, and so on. This variety in the endowments of the cells, by whose aggregation and conversion the fabric of the higher Animals is made up, is a fact which we cannot explain, and which must be regarded (for the present, at least), as one of the "ultimate facts" of Physiological Science.

716. Passing by the extended membranous surfaces, and the protective cells with which they are covered, we find that the simplest form of a secreting organ is composed of an inversion of that surface into a series of follicles, which discharge their contents upon it by separate orifices. Of this we have an example in the gastric follicles, even in the higher animals; the apparatus for the secretion of the Gastric fluid never attaining any higher condition, than that of a series of distinct follicles, lodged in the walls of the stomach, and pouring their products into its cavity by separate apertures. In Fig. 107 is represented a a portion of the Ventriculus succenturiatus of a Falcon; in which the simplest form of such follicles is seen. A somewhat more complex condition is seen in some of the Gastric follicles of the Human stomach (Fig. 80); the surface of each follicle being further extended by a sort of doubling upon itself, so as to form the commencement of secondary follicles, which open out of the cavity of the primary one.-Now a condition of this kind is common to all glands, in an early stage of their evolution; and in this stage, we meet with them, either by examining them in the lowest animals in which they present themselves, or by looking to an early period of their embryonic development in the highest. Thus, for example, the Liver consists, in certain Polypes and in the lowest Mollusca, of a series of isolated follicles, lodged in the walls of the stomach, and pouring their product into its cavity by sepa rate orifices; these follicles being recognised as constituting a biliary apparatus, by the color of their secretion. And in the Chick, at an

early period of incubation, the condition of the Liver is essentially the same with the preceding; for it consists of a cluster of isolated follicles not lodged in the walls of the intestine, but clustered around a sort of

bud or diverticulum of the intestinal tube, which is the first condition of the hepatic duct, and into which they discharge themselves (Fig. 108). So, again, the Pancreas first presents itself in the condition of

a group of prolonged follicles, or cæca, clustered round the commencement of the intestinal tube; and this is its permanent condition in many Fishes (Fig. 109). And the Mammary Gland possesses an equally simple structure in the lowest of all the Mammalia (to which group it is restricted); namely, in the Ornithorhyncus (Fig. 110).

717. The next grade of complexity is seen, where a cluster of the ultimate follicles open into one common duct, which discharges their product by a single outlet; a single gland often containing a number of such clusters, and having, therefore, several excretory ducts. A good example of such a condition, in which the clusters remain isolated from one another, is seen in the Meibomian glands of the eyelid (Fig. 111); each of which consists of a double row of follicles, set upon à long straight duct, that receives the products of their secreting action Fig. 111.

Fig. 112.

and pours them out upon the edge of the eyelid. And of the more complex form, in which a number of such clusters are bound together