the investing membrane is in many instances simply formed by the condensation of the con necting cellular tissue, but in the larger glands a proper fibrous capsule is provided, which adheres more or less intimately to the proper glandular texture.
General conclusions respecting the minute structure of glands.
1. That throughout the whole range of the animal kingdom and in every species of gland there is one uniform type, from which the glandular formation in no instance deviates.
2. That every gland consists of a membrane derived either from the skin or mucous mem brane.
3. That this membrane is disposed either in the form of a pouch or of a tube more or less ramified, and terminates in every instance, with out an exception, in a blind extremity.
4. That the secreting canals are most diversi fied in form, being simply sacculated, branch ing, pennitifid, nail-shaped, or enlarged at the commencement, cellular, berry-shaped, serpen tine.
5. That granules or acini in the hypothetical sense of writers do not in reality exist.
6. That whatever may be the variety of form it is always subordinate to the grand principle which the whole glandular system displays,_ namely, that the largest possible extent of secreting surface is contained in the smallest possible space.
7. That there is no immediate connexion or continuity between the secreting canals and the sanguiferons vessels.
hypotheses respecting the minute structure of glands.-1 was desirous in the first part of this article to convey to the reader a com prehensive view of the glandular structure, uninctimbered by any reference to the opinions of anatomists on this subject; but the hy potheses of Malpighi and Ruysch have so long divided the world of science, that it is necessary to ascertain how far the doctrines advocated by those celebrated men are in accordance with the above-stated conclusions. In doing this, however, much difficulty is experienced, especially in considering the opinions of Malpighi, inasmuch as his com parisons of the minute structure of the liver, of which organ he principally treated, are very vague and obscure, and being for the most part unaccompanied by illustrative plates, it is almost impossible in many of his descriptions to detect the meaning he wishes to convey. But, notwithstanding these obstacles, it is evident, on studying his account of the liver and kidney, that justice has not been done to his researches ; for he not only corrected many of the then prevailing errors, but also ascertained several important points connected with this interesting branch of anatomy.
Malpighi compares the minute lobules of the liver and other conglomerate glands to a bunch of grapes, these lobules being joined to the neighbouring lobules by intermediate vessels. His words are, " for as an entire bunch of grapes is formed of small bunches by a communion and tying together of vessels, which small bunches are themselves formed into a mass by appended grapes (acini); so the whole liver is formed by lobules many times folded, and which are themselves formed of glandular globules." It is thus observed that Malpighi describes in the liver larger and smaller lobules ; and it is to these latter that the celebrated but vague term of acini appears to be more particularly applied. lie observes that the lobules are of various forms in different animals ; in fishes having the shape of a trefoil, in the cat six-sided, &c. The inter lobular spaces are noticed as being distinct in fishes, but as obscure in the more perfect animals.
With respect to the intimate structure of the small lobules, or acini, Malpighi conceived that each of them consisted of a hollow vesicle, receiving the secreted fluid from the small arteries and conveying it into one of the roots or branches of the hepatic duct ; or, in other words, that the structure of the acinus was the same as that of the simple mucous follicle.
Owing, however, to the imperfect means then possessed of prosecuting such inquiries, it is certain that Malpighi did not detect the ultimate structure ; for more exact observations have proved that the last divisions of the secreting canals, although they constantly terminate in meal extremities, do not always end in follicles, but that they may consist of serpentine tubes, as in the kidney, or of pennatifid canals, 8cc. It also has been determined that what he regarded as the last divisions of the ducts, or acini, are themselves composed of smaller canals. But his observations on the develope ment bf the liver in the chick shew that he was acquainted with the essential facts con nected with the structure of that organ, and with the mode of its formation ; for among other interesting remarks, he says that on the seventh day of incubation the liver of a yellow ish or ashen colour presents granules of rather an oblong form, and "as it were blind pouches, appended to the hepatic duct."1 This hypothesis, founded as it is on so large a body of evidence, was generally re ceived ; but the discovery of the art of minute injection, which seemed to afford ocular de monstration of the fallacy of Malpigbi's theory, induced the majority of anatomists to adopt the ideas rather pompously announced by Ruysch. This celebrated anatomist, rejecting the hypothesis of Malpighi, contended / that he had proved, by injection, that the arteries are directly continuous with the excretory ducts; or that the little ducts proceed from the minute arteries, like lesser from larger branches; and that each acinus consists prin cipally of bloodvessels, but contains also an excretory duct.§ In considering the merits of these two hy potheses, it becomes apparent that ltuysch supported his opinion by evidence of a most insufficient character ; for in investigating the most complicated glands he relied solely on his vascular injections, to the exclusion of the evidence afforded by the much more satisfactory researches of comparative and developmental anatomy. lf, as Professor has ob served, Ituysch had carefully examined his injected organs with the microscope, he would have found that between the most delicate plexuses of the bloodvessels there is always an additional substance destitute of vessels ; although these organs, when seen by the naked eye, appear to be stained in every direction with the coloured injection." But even ad mitting what frequently happens from a too forcible injection, that the matter thrown into the arteries is found in the ducts, this does not prove that the small bloodvessels are con tinuous with the excretory canals; for after the sanguiferous vessels are filled, they easily become ruptured, and so allow their contents to escape into the ducts. It may further be objected, that in all the glandular organs which have been carefully inspected the commence ments of the excretory ducts arc larger than the least arteries;-1- indeed, Ituysch's own account of this imaginary continuity is very vague, and the plates designed to illustrate his theory, especially that of the kidney, are any thing but satisfactory. As Ituysch not employ the microscope, it is impossible he could have seen that continuity which he so confidently described ; indeed, as Haller remarks,] it is difficult, or rather as we should say impossible, to demonstrate, with the aid of the most powerful lens, the connexion of the last arteries with the coats of the ducts.