We are therefore correct in regarding the supra-renal capsules as glandular organs, and their function as secretory ; and any hypo thesis which ascribes to them a function other than secretory, may be safely considered erro neous.
If we now address ourselves to a comparison of the structure of the supra-renal capsules with that of other vascular glands, we shall see how great a resemblance obtains.
Unfortunately, we do not possess such ex perimental researches regarding all of them as those which Ecker has published in his work on the supra-renal capsules. Only the thymus gland has found an accurate observer in Simon.* Nevertheless, we already know thus much, that in two of the three other vascular glands, viz., in the thymus and thy roid glands, a similar structure obtains. Bar deleben t was the first to observe what Gerlach §, Schaffner II, and others confirmed, that in the thyroid gland gland vesicles occur grouped in masses, imbedded in a loose cellular tissue, and surrounded, by blood-vessels. In Man, they are seen with some difficulty ; but are more easily made out in most of the Mammalia, and best in Birds and Iteptilia. These vesicles, which attain a size varying from 6-1000ths to 5-100ths of a line, begin as cells which mea sure 5 to 10-1000ths of a line, and which are connected with them by all the transi tional forms. Suspended in a fluid which occupies their interior, are varying numbers of elementary granules of an albuminous and besides these, fatty granules, nuclei of 2 to 3000ths of a line, and finally, cells with one or more nuclei. in the earlier periods of fcetal life the vesicles are not to be met with ; but their place is entirely oc cupied by cells, — a condition exactly similar to that which was met with in examining the supra-renal capsules.
So also the gland-vesicles of the thyroid appear after some time to be destroyed, and their contents resorbed. This is shown by the fact, that frequently only younger and undeveloped forms of these are to ''be met with in the adult animal ; while in other in stances, the fully developed vesicles occur in excessive numbers. Some distinction between the thyroid and supra-renal capsules, although not an important one, may possibly be de duced from the fact, that in most of the Mam malia, the nuclei and cells are ranged as an epithelial stratum on the inner surface of the vesicles, so that the remainder of their cavity is filled with a more or less clear fluid. Never theless, in Man, the gland-vesicles are generally completely filled with the morphological con stituents of their contents.
As regards the thymus, we have lately had Simon's researches extended by Ecker, Ger lach, and Schaffner. The gland-vesicles here likewise enclose similar contents. The quan tity of the elementary granules seems to be very variable, and nuclei form the greater part of the contents. In general, cells occur but very sparingly. According to Simon, their number is increased by involution of the organ, in which case, also, fat molecules are wont to appear in place of the elementary granules. As to the gland-vesicles, they deviate from those of the vascular glands hitherto described. In the embryo of the Adder and of Birds, Ecker saw many completely shut gland-vesicles, which everywhere passed into sacculi by pouchings of their menibrana propria. In the human thymus, and that of Mammalia generally, only a few vesicles of this kind were found, and the tunics of these exhibited the same condition. Thus the thymus gland exhibits a similarity with the clustered glands ; but is distinguished from these by the want of an excretory duct. And these differences of the gland-vesicles, toge ther with the great number of the nuclear structures, somewhat distinguish the thymus from the supra-renal and thyroid glands.
The structure of the spleen is even more different,— if it be allowable to decide upon this organ, which is so uncommonly diffi cult of examination, from a few observations of a somewhat contradictory tendency. It is only the well-known Malpighian corpuscles which can be regarded as glandular struc tures. In fact, this opinion would seem to be correct, since Ecker found a delicate mem brane enclosing the vesicles. Nevertheless,
lately, an opinion has been put forth on many sides, and based on continuous research, that the Malpighian corpuscles are not closed vesicles, but stand in connection with the lymphatics, so as to constitute only vesicular dilatations of these vessels. This view, which is maintained by Gerlach and Schaffner, is especially corroborated by the fact, that the contents of the splenic vesicles differ from those of the other vascular glands, and closely approximate to those of the lymphatic ves sels. Thus we remark in their contents, cell nuclei in very predominant quantity, the dia meter of which amounts to 2-1000ths of a line. These nuclei appear sometimes smooth, sometimes granular ; they often exhibit one or two nucleoli, while in other instances they have none. Roundish cells of 4 to 5-1000ths of a line, and closely surrounding a nucleus, may also be remarked in sparing quantities. The above-mentioned relation of the Malpi ghian corpuscles to the lymphatic vessels is probably supported by another circumstance. Another cell formation may be found in al most every spleen, and certainly in the splenic vesicles also. They are large, globular cells, ffieasuring from 5 to 10-1000ths of a line, and with contents of a very different kind; so that from this fact one might conjecture that they form a number of very different struc tures, which, however, are connected with each other by transitional forms. Thus a part of these cells contain more or less dark elementary granules ; another part enclose larger yellowish, or yellowish brown and tuberculated corpuscles ; while finally, other cells contain, with the latter corpuscles, one or many completely developed blood corpus cles. This group of cells is susceptible of a double interpretation. In the first place, effused blood corpuscles may be enclosed by a cell membrane, and may be broken up into elementary granules in its interior. This view has been taken by Kolliker*, and is supported by pathological appearances. But the process may be exactly the reverse ; the blood corpuscles may be developed in the interior of the cells, and then, after the burst ing of the cell wall, become free. And should this latter view, which is especially maintained by Gerlacht, ultimately turn out to be correct, then we shall have in the Malpighian vesicles of the spleen a new development of blood corpuscles, just such as occurs after another scheme, in the other district of the lymph and chyle system. So that the spleen would be an organ especially serving for the develop ment of blood corpuscles, and the old ex planation set forth by Hewson would be the correct one. But if the process be that which Kolliker supposes, the spleen will exercise the function of destroying the blood corpuscles. In any case, it is worthy of notice, that no such cells containing blood corpuscles occur in the other vascular glands. Gerlach could not discover them in the thymus gland, and they would scarcely have escaped Ecker's careful observations of the supra-renal capsules. So likewise, in a number of examinations of supra-renal capsules, it is impossible to find the least trace of their presence. Therefore by all this the spleen is greatly distinguished from the other vascular glands. Such great differences of structure and contents will not allow us to attribute a like function and a like secretion to all these organs. Certainly we cannot at all state of what kind these differ ences of secretion are, since the materials prepared by these vascular glands are, as far as regards their composition, completely ne glected : but that such differences must exist, I think microscopic research will quite warrant us in saying. In this respect the thyroid gland and supra-renal capsule agree most nearly together, since in general terms, there is a great correspondence in their struc ture. And, as the spleen is the widest se parated from them, so it is not unlikely that future research may remove it from amongst the vascular glands.