In the interior of the organ itself these small arterial trunks still take a different course. One of these sets of vessels can only be followed for a very short extent in the cortical substance. After a course of scarcely half a line in length, they break up into a capillary network, with long meshes, which encircle the gland vesicles of the cor tical substance. The other arterial trunks, without giving off any branches, plunge at right angles through the cortical mass into the medullary substance. Here they break up into twigs which, by devious paths, return again into the cortical mass, to end also in a network of capillary vessels. Thus the cor tical substance possesses a closer capillary network than the medullary mass, a fact which is in conformity with the predominant glan dular activity of the former stratum. Nagel, in his Essay, has illustrated this distribution of the arterial vessels by beautiful drawings.
The veins begin at the common margin of the cortex and medulla. They alone con stitute almost all the vessels of the latter por tion, since the smallest branches gradually unite to form larger ones, and these finally join to form the vena supra-renalis, which occupies the middle of the supra-renal cap sule; and, as was stated above, is proportion ally very large. " Were we able," says Nagel, " to exhibit this venous texture isolated from the cellular tissue which receives it, the indi vidual smaller veins opening into the venous trunk on all sides, and at very acute angles, would render it most suitably comparable to a poplar tree." Usually there is a single vena supra-renalis for each supra-renal capsule. The right supra renal vein passes immediately from the gland into the vents cava ascendens ; but the left one forms a longer trunk, which opens into the renal vein. But these statements of Nagel are contradicted by others ; for instance, by Krause, according to whom two or three veins generally leave each supra-renal capsule.
As far as observations have hitherto gone, this disposition of the vascular system appears to be tolerably constant for all the Mam malia.
The lymphatics of the supra-renal capsules are not yet sufficiently known. According to Arnold, there are superficial and deep ones. In animals slain during the digestive act, Ecker could only find the superficial lym phatics, so that Arnold's statements would seem to be doubtful. The absorbent trunks of the supra-renal capsules unite with those of the kidneys and the internal sexual organs, and they open into the thoracic duct.
The uncommonly rich supply of nerves possessed by the supra-renal capsules consti tutes a very striking phenomenon, and one i which finds no parallel in any cognate organs. In Man, these nerves arise from the cliac and renal plexuses in the form of numerous and proportionally large trunks. It is chiefly
through Ecker that we have become ac quainted with the further course of the nerves in the interior of the organ. Larger and smaller branches of nerves perforate the cor tical substance, usually contained in the bun dles of its areolar tissue, or accompanying its arterial vessels, but giving off no primitive fibres. Only in the medullary substance do the trunks of nerves break up into bundles of fibres. The neighbouring bundles often ex change their nerve-fibres with each other, so that, in this way, they form a dense micro scopic nervous tissue. It would seem that, in the human subject, no ganglion corpuscles are present in this sympathetic nerve tissue ; but, in the Horse, in whom the supra-renal corpuscles are yet richer in nerves than they are in man, some ganglion corpuscles may be seen on the nervous trunks of the tissue. But one must he especially careful against confounding gland-cells of the organ with these ganglion corpuscles, which they super ficially resemble. This rich supply of nerves seems only to pertain to the supra-renal cap sules of the Marnmalia ; in all the other Ver tebrata, whose supra-renal capsules, as will be forthwith stated, are very similarly constituted, it is completely wanting. On this account, it becomes exceedingly difficult to state what object is fulfilled by this united condition of the nerve fibres.
In Birds, the structure of these vascular glands is even more uniform than in the pre ceding classes. No separation into cortical and medullary substance appears to occur in any bird. A single observation of Meckel's states that in the Cassowary the two kinds of substance are to be distinguished ; but this statement is contradicted by the numerous negative observations of Cuvier, Ecker, Bar deleben, and Nagel, on all the animals of this class which they have examined, and hence it requires a confirmation. The supra-renal capsules of birds appear of a darkish or clearer yellow colour, which, by the varying amount of blood they contain, tends more or less towards red. They are also enclosed in a sheath of areolar tissue, and exhibit a basis which consists of fibrous tissue and blood • vessels. In this matrix the gland-vesicles lie imbedded throughout the whole thickness of the organ, and they are devoid of any regular arrangement. They are of roundish, oval, pear-shaped, or irregular form. Their size varies from 44 to 56-1000ths of a line, and in the Falco tinnunculus some of them even attain the size of 22.100ths of a line. Their membrana propria is very delicate. The structure here specified may be best wit nessed in the supra-renal capsules of large birds ; nevertheless, with some care, we may succeed in recognising the gland-vesicles in the smaller members of the class, as, for instance, in the Pigeon.