We will, therefore, only attempt to answer such physiological questions concerning the supra-renal capsules which are possibly sus ceptible of solution, and defer their full under standing to the future.
We will see whether the supra-renal cap sules in general can be called glands, and how far they correspond with the other glands of blood-vessels, with the thyroid, thymus, and spleen. And then we may answer the ques tion, " whether the supra-renal capsules ge nerally eliminate a glandular secretion, and whether this secretion is to be regarded as similar or diverse in the whole of the vascular glands ?" Microscopic anatomy has taught us to re• cognise three kinds of form under which glands are developed in the higher animals. The first is a fine tube or canal of different length ; the second is a small globular or elongated sac, which is open at one end ; and the third form is that of a globular and com pletely closed vesicle. A very delicate struc tureless membrane, the menibrana propria, forms the frame-work of all these glands ; and it is distinguished by its insolubility in dilute alkalies. On its exterior, it is surrounded by a network of capillaries, while it includes within its interior, glandular contents in very diverse forms. These contents consist, in general, of elementary granules, of an albu minous material, of fatty granules and small drops of fat, and of nuclei and cells in the most variable quantities. Cells, which are the highest development of glandular contents, are not always found : more frequently only nuclei are noticed. It will be recollected that in speaking of the contents of the supra-renal capsules, we have stated that the development of cells occurs by the method of cireumposition of a globular membrane, and the same method obtains in a number of the glands.
The first two of the three forms of glands frequently occur isolated, and thus exhibit microscopic glandules of the greatest sim plicity ; as for instance the stomach glands of Mammalia, the glands of Lieberkuhn, the cu taneous glands of the Frog. In other instances, glandular tubes or sacs are connected to each other in very variable number, and thus form the larger glands. In this way the kidneys
and testicles originate from tubes ; while from sacculi are formed the multitude of clustered glands, as those of Brunner, the salivary glands, and the pancreas.
The first two forms of glandular structure have no further interest as regards our present object ; but the case is very different with re spect to the third or vesicular form.
The gland-vesicles have a very diverse size, ranging from microscopic dimensions to a quarter, a half, or even a whole line or more. Their shape is always spherical, so that their membrana propria encloses a globular cavity, which contains, suspended in a fluid, all the different constituents of gland-contents pre viously spoken of. In very numerous experi ments I have never found vesicles of this kind further divided by ensheathing membranes. • Generally the united gland-vesicles are in. laid in the fibrous tissue of the particular mucous membrane of the body ; they are sometimes in greater, sometimes in smaller quantity, so that great differences, both in dividual and generic, may be noticed in this respect. In many places they occur with greater frequency, so as to constitute the rule, and have therefore received a special name. For instance, the glandulee solitarim of the intestine, and the lenticular glands of the mucous membrane of the stomach, belong to this category ; while in other cases more than one or even many vesicles of this kind are grouped together to form a glandular body of some extent. Amongst such aggregations may here be enumerated the Peyerian vesicles, which form a single layer of this kind in the mucous membrane of the intestine. We find the same grouping in the ovary, where these vesicles are called " Graafian," in honour of their discoverer. But here they are some what more widely separated from each other by the thickness of a solid fibrous basis, or by the so-called stroma ; and they enclose, beside the usual glandular contents, a peculiar body, the primitive egg or ovum.