It is highly singular that the human supra renal capsule in the earlier period of foetal life, is not only much larger in proportion to the size of the body than it is at a later stage, but that it also considerably exceeds the kidney in size (fig. 543.). This is shown by numerous observations of Meckel, Midler, and others. In a human embryo of the ninth week, Ecker has recently made the same ob servation. In the embryo of the tenth to the twelfth week, which measures about two inches in length, the kidneys and supra-renal capsules are for the first time of pretty equal size. While, on the other hand, in the sixth month the kidneys are, as was found by Meckel, already about double the size of the supra renal capsules, and their weight is to that of these latter as five to two. And as this com parative diminution of the supra-renal cap sules proceeds further, the proportions by weight are in the mature embryo as three to one. And even beyond this time, they expe rience a continual diminution, so that finally, in the adult, the supra-renal capsule is only 1-28th the size of the kidney, Strange to say, this extraordinary size of these organs during the foetal period, does not obtain in the other Mammalia. In the earliest period of their embryonal life, the kidneys have always been found considerably larger than the supra-renal capsules. In them both organs appear to grow in precisely equal pro portion to each other, so that their (compa rative) weight remains the same in the adult animal as in the foetus. According to Ecker, the supra-renal glands of the new-born kitten are 1-56th to 1-60th the size of the urinary glands. So also the other classes of Vertebrata seem to accord with the Mammalia in this re spect. At least Ecker observed, in the em bryo and adult animal of the Coluber natrix, the same relative proportion of size between the kidney and supra-renal capsule.
If we regard the situation of the supra-renal capsules in the animal kingdom, we shall find a different condition in the different classes.
It is well known that the Wolffian bodies begin in the embryo at a very early period, and that these, from their structure and func tion, must be regarded as the kidneys of the embryo ; so that the name of " primordial or original kidneys," may be applied to them with all accuracy. Now these primordial kidneys have a very different duration in the different classes of Vertebrata. In Fishes they remain as the urinary glands during the whole of life; since in them, as Baer and Rathke found, an organ similar to the kidneys of the higher Vertebrata is never developed.
The kidneys of Fishes are, therefore, pri.. mordial kidneys.
But in the three higher classes of Vertebrata the fact is found to be otherwise. Here the primordial kidneys pertain to only a part of the foetal life, and then make way for the true kidneys, which are situated beneath the supra renal capsules.
Therefore, the supra-renal capsules of fishes are placed on the Wolffian bodies, while in the other Vertebrata they are in connection with the kidneys.
In Man, in whom the supra-renal capsules are at first uncommonly large, the primordial kidneys disappear remarkably early, so that in the second month they have vanished even to the smallest relics.
The development of the tissues of these organs is as yet most incompletely known ; at present we have only a single statement of Ecker's respecting it. In a human embryo of twelve weeks, this anatomist found a very distinct cortical and medullary substance. But only part of the morphological consti tuents were present ; viz., the elementary granules, fat, and nuclear structures. Cells, with or without a membrane, were only sparingly found ; and gland-vesicles were altogether absent. The embryos of other Mammalia exhibited the same results. In the foetus of an ox of l feet in length, the vesicles were for the first time detected ; and in this instance, as in those of the mature animals previously described, the gland-vesicles were, according to Ecker's opinion, developed from enlarged cells.
In any case it is exceedingly interesting to observe, that in spite of the very early found ation of the supra-renal capsule, this organ seems to rest for a considerable time ; since, before their gland-vesicles are developed, we cannot ascribe any activity to the supra-renal i capsules. But in man, these vesicles are
only developed when the period at which the supra-renal capsules attain their greatest bulk is long passed.
IV. Physiology.— Finally we apply our selves to the functional relations of the supra renal capsules. Unfortunately, as regards the activity of the glands of blood-vessels, they all veil themselves in an impenetrable ob scurity. The recent vast strides of Physiology have passed over them without leaving any traces ; the last ten years having only brought some new hypotheses concerning the function of these organs, and very doubtfully enriched the already great number of older conjectures.
It is not the aim of this essay either to answer or to enumerate all that physicians of earlier and later times have imagined as the possible function of the supra-renal capsules. These conjectures, for the most part ephemeral, may be properly left to the history of ana tomico-physiological science ; and since we willingly disclaim the intention to come for ward with a new hypothesis, we will only call attention to some points of this mass of matter.
Now that the minute structure of the supra renal capsules is known, it becomes even more important to dissent from the opinion of Cuvier. He attributed to the supra-renal capsules pretty much the same function as the kidneys," because these two organs have much similarity with each other, both in their form, and in the tissues of which they are com posed." At present we know how great is the diversity between the tissues of the two organs. Besides, the want of an excretory duct is another fact completely passed over by Cuvier. The opinion of Meckel, that the supra-renal capsules stand in dependance with the sexual functions, rests on just as little foundation, and has been already confuted at length by Nagel in his work. So also the conjecture of Bergmann* and others —accord ing to which the supra-renal capsules stand in a very close relation to the nervous system of the foetus — is equally erroneous. We will willingly admit the correctness of the observ ation, that in brainless monsters the supra renal capsules are generally deficient ; but we are forced to add, that we cannot annex any distinct notions to this close relation to the nervous system. Besides, this would suppose that the function of the supra-renal capsule differed in the foetus and in the adult animal ; since certainly no one would seriously defend such a relation of their function in the latter case. And the support which this view seems to receive from the structure of the supra-renal capsules, is likewise only a fallacious one. At one time, attention was drawn to the great richness of the supra-renal capsules in nerves, without reflecting that this is the case in only one out of the four classes of vertebrate ani mals: and in the same way, the superficial like ness possessed by the gland-cells and ganglion corpuscles was adduced in order to make out the supra-renal capsule as a kind of ganglion. This hypothesis was put aside by the dis covery of the gland-vesicles. And especially we would briefly remark, that all those theories which ascribe to the supra-renal capsules a special import during embryonal life, have ori ginated from contemplating them in the human foetus, in whom they are exceedingly large. Had the supra-renal capsules of other animals been kept in view, those opinions could scarcely have been taken up ; and the special size of these organs in the human embryo would have been recognised for that which it really is, an anomalous, isolated condition, which pos sibly stands in a causal relation with the swift disappearance of the Wolffian bodies. And, as we have already remarked, even apart from this, the absence of the gland-vesicles from the supra-renal capsules of the small embryo is a fact which must not pass un noticed.