- VA" OD Nt • • 7.
creting membrane is, for the sake of conveni ence, disposed in the form of extensively rami fied tubes, it is most important to recollect that the glandular fluid is poured not only into the ccecal extremity or commencement of each tube, as is the commonly received opinion, but along the whole extent of the tube. For the establishment of this fact, certainly the most important in the history of the glands, we are principally indebted to Professor Mbller.
Excretory duct.—Although the essential seat of the glandular function is now ascertained, some difficulty exists in determining the actual extent of the secreting surface in the various organs ; or in other words, at what precise point the mucous canals ceasing to secrete, become mere excretory passages.t An attempt to decide this point is, however, necessary, because until this time the majority of anato * Berres, 1. c. pt. 5, tab. ix. fig. 2.
I allude here of course to the peculiar secre tions, as the bile, urine, milk, &c. and not to the secretion of mucus, which we know is poured out along the whole extent of the excretory ducts.
mists have signified by the term excretory duct, not only the canals which simply bear away the secreted fluid, but likewise those tubes which constitute the true secreting apparatus, and which, it is evident, are at the same time both secreting and excreting canals, as they not only secrete, but likewise carry to the larger and non-secreting ducts the fluids poured out by their parietes.
In the simple sacculi or follicles, it is evi dent that the secreting structure is co-equal with the extent of the bag itself, so that the little orifice becomes the excretory duct; in the tonsils, prostate, &c. there are several such orifices or ducts. But at what point does secretion cease in the compound glands ? Mr. Kiernan states,* that in the liver the secreting portion of the organ is confined to what he calls the lobular biliary plexuses, or to those tubes which are placed within the lobules; so that here the excretory apparatus is very com plex, consisting of the interlobular tubes, those • Phil. Trans. 1833, p. 741.
of Glisson's capsules, and, lastly, of those which quit the organ. The limitation thus established, and which I have no doubt is strictly correct, may be applied to all those glands, such as the salivary, mammary organs, Scc., which, like the liver, possess distinct lobules. In the kidney the true secreting structure i3 probably restricted to the serpen tine tubes contained in the cortical texture, (canalcs corticalcs, or ducts of Ferrein,) the straight canals ( lab.'Belliniani) constituting the cones, which bodies, in a minute injection of the hloodvessels, are nearly colourless, being merely for excretion.* The testis, with its appendix the epididymis, presents an in tricate arrangement; it is probable, however, that the principal secreting part consists of the seminal tubes which form the lobules, and that the vasa recta and efferentia are merely ex cretory in their office; in the upper part of the epididymis a second secreting structure is met with, constituting the cord vasculosi, whilst the lower part containing the convolutions of the vas defercns is of the excretory character.
It is proper to observe that the process of secretion is incessantly going on ; but with the exceptions of the mucous, sebaceous, and a few other glands, the fluids produced are destined to be poured out only at stated in tervals; it is, therefore, evident that some con trivance is required, by which the several secre tions may be retained, till the moment arrives when it is necessary they should be discharged. The liver may be selected to illustrate this principle: one of the most essential functions of that organ being the decarbonization of venous blood, its constant action is no less indispensable, indeed considering the whole animal series, is even more indispensable than that of the lungs themselves; and yet the pro duct of that action, the bile, is only designed to be poured into the duodenum during the process of digestion. In order to obviate the irritation of the bowels that would result from the incessant discharge of the bile, and at the same time to economise that fluid, the gall bladder is provided, which, receiving the se creted fluid in the intervals of digestion, fulfils all the conditions required. The absence of the gall-bladder in several classes of animals can scarcely be admitted as being incompatible with this explanation ; fur the majority of these instances of deficiency occur in non ruminant vegetable feeders, in several genera of the Pachydermata and Itodcntia for example, in which it is evident that as the process of digestion must occupy a considerable period, a prolonged flow of bile is requisite, and a special reservoir is less necessary; in addition to which it is known that in some of these cases, as in the horse and elephant, the principal trunk of the biliary ducts is very large, and may in some degree supply the place of a gall bladder' The urinary bladder is a provision rather of convenience than of necessity, enabling the animals that possess it to retain the urine as it flows from the ureters, until a considerable accumulation takes place. These are the only instances in the human body of a distinct ro servoir being provided ;1- but every gland, by retaining its secretion in the excretory ducts, has a power of emitting the fluid, under certain circumstances, in larger quantities than usual, as in the case of the salivary and lachrymal glands; a similar accumulation must take place in the seminal tubes and prostatic ducts, and especially in the lactiferous tubes and their terminal sinuses. In animals the examples of distinct reservoirs are too numerous to be here enumerated.