The fluid of the thymus-cells contains (in the active period) proteinecompounds and traces of fatty matter; and the gland itself, on ultimate analysis, yields nearly the same proportion of essential elements as blood and flesh do.

The most important facts afforded by the comparative anatomy of the thymus, laboriously investigated by Mr. Simon are, 1st, that it "belongs, without exception, to all animals breathing with lungs, and to no others." 2. That in the hybernating rodent animals, the large persistent thymus and its prolongations into large masses which lie in the posterior mediastinum, are wholly composed of fat, the nuclei or cytoblasts which its cells contained having, at the approach of the hybernating period, genuine fat-cells formed round them. 3. That the thymus of birds, hitherto unknown, is a semitransparent ampullated tube, following the line of the superficial cervical vessels, and very early ceasing to discharge its functions. 4. That in reptilia, to which also a thymus was generally denied, its existence is constant. In the hybernating serpents also there is always appended to the persistent thymus, an accessory organ or fat-body. In the batrachia and some others, the thymus is at a very early period converted into a mass of fat, which is persistent. The larvæ of batrachia while breathing with gills have no thymus; its development begins as soon as their pulmonary respiration is established. Among the icthyoid reptiles with persistent gills, it is found in the menopoina, amphiuma, axolotl, and menobranchus, but not in the siren and proteus. 6. In fishes, a thymus is not found.

Thyroid Gland. According to Mr. Simon, the thyroid gland consists of a dense aggregation of completely closed vesicles, each formed by a layer of delicate homogeneous membrane (limitary membrane), invested by a close capillary network. These are the analogues of the cells or follicles of the thymus, but they are completely closed, and do not communicate with any central cavity. They are filled by fluid and cytoblasts, which are the analogues of those in the cells of the thymus; and are held together by fibro-cellular tissue. The cytoblasts are not materially different from those of the thymus; in the young animal they lie close together on the inner surface of the containing cell, like, Oesterlen says, "a tesselated epithelium;" but from this position they detach themselves and float freely in the fluid of the cavity. That they have the relation of nuclei is proved, according to Mr. Simon, by their being not unfrequently found as the nuclei of cells, about of an inch in diameter. He shows also that a thyroid, or an organ representing it, exists in all vertebrate animals, appearing to have relation to the development of their nervous centres, always maintaining an intimate relation to the vascular supply of the brain, existing in certain fishes as a mere diverticulum to the cerebral circulation, and in the animals above them having a super-addition of glandular structure.

Spleen. Oesterlen and Mr. Simon agree that the Malpighian bodies of the spleen are not vesicles, but aggregations of cytoblasts (analogous to those of the vascular glands already described) which are herein collected in small bodies; but these have no inclosing cell wall; each lies within a kind of capsule of capillary vessels, receiving themselves no vessels into their interior. They are held together by an amorphous transparent substance with obscure fibres; among which Oesterlen believes he once detected lymphatic vessels. He has not always found the Malpighian bodies in any of the animals in which he has examined them. Most of the corpuscles or cytoblasts of the spleen, whether scattered through its red substance, or collected in the Malpighian bodies, appear like those of the thymus, but they vary in their contents; others are larger and are merely close-pressed aggregates of granules; others are partially surrounded by darker granules; and others, in different species, differ from all these, approaching the characters of blood-corpuscles.

Renal Capsules. Oesterlen has described the structure of these more minutely than that of any other vascular gland. He finds inconstancy in the distinction, proportions, and colours of the cortical and medullary substances. His account of the blood-vessels accords with Müller's. He finds no central

cavity (except that of the great central vein); but, occasionally, hollow, elongated spaces of conical shape, which have no lining membrane, and are empty, or contain a thick grayish-white fluid. The appearance of radiating striæ in the cortical substance of these organs is due to groups of the small corpuscles or cytoblasts of which these, like the other vascular glands, are chiefly composed; and of which, in these, many are grouped together with fat-cells and molecules in the form of nearly parallel cylinders or elongated cones; each group being, as Mr. Simon has discovered, inclosed in a tube of very delicate membrane. The medullary substance of the renal capsules, and that which intervenes between the tubes full of corpuscles consist, according to Oesterlen, of cytoblasts, uniformly scattered, and with these are mingled minute molecules, and small collections of fat-cells or particles.

The various microscopic objects found in the renal capsules are, according to both Oesterlen and Simon—(a) minute oil or fat particles, either scattered or grouped in round, oval and retort-shaped flattened masses; (b) similar groups of fat particles collected round the proper cytoblasts of the organs, but devoid of separate cell-walls; (c) the cytoblasts, pale, roundish or oval, disc-like, or more commonly, concavo-convex some have but the appearance of a nucleolus ; in others it is distinct, central, or scattered, or marginal; (d) similar cytoblasts surrounded by oval cells; (e) four other forms are enumerated by Oesterlen, which are probably only varieties of the preceding.

The preceding statements justify the conclusion drawn by both the authors that these four organs are so similar, that they may be classed as members of one order. To say nothing of their likeness in obvious characters, the essential constituents of all, their peculiar corpuscles, are similar in all, though admitting of distinction in their best marked states. In all, these corpuscles have the relation of cytoblasts or nuclei. In all, some of them are collected in groups of definite, though various form; which groups are in all except the spleen, inclosed in vesicles or tubes formed by delicate membrane, (primary, limitary, or basement membrane), the exterior of which is covered by capillary blood-vessels. Oesterlen, who cites, besides these analogies, many others of less importance, includes, in the same class with these four organs, the lymphatic glands, the pineal and pituitary glands, the choroidal gland in the eye of fish, and the greenish gland on each side of the gastric sac in crabs, and suspects that many structures in invertebrata will be brought into the same class. In the lymphatic glands he finds, besides those of the lymph, a variety of corpuscles like those in the renal capsules. They may be generally distinguished into two kinds; viz., fat cells and molecules, as in the renal capsules, and the cytoblasts [which, doubtless, are the same as those composing the layer of granular matter described by Mr. Goodsir]. These cytoblasts are various, but the majority cannot be distinguished in any way from the cytoblasts of the thymus. Moreover, Oesterlen says that he has found, in an inguinal gland of a fœtal calf, gland-vesicles like those of the thymus, parotid, &c., formed by transparent membrane, with traces of capillary network on it, and full of cytoblasts, with which fat-molecules and fat-cells are mingled.

Oesterlen describes a very similar structure in the pituitary gland: and he says the pineal gland is similarly composed.

But to know the number of organs which may be included in this class or family, is less important than to determine to what other class of organs they are in nearest relation. It is clear that (as many have believed, but none have proved), they are, in all essential characters, glands, and that the name of either vascular glands, or glands without ducts, is appropriate to them. Their chief analogies to the true glands are seen in (a) the constant possession of cytoblasts, or nuclei, analogous to the nuclei of the true gland-cells; for even in the latter it is probable that the nuclei, rather than the cells, are the most active and essential apparatus; (b) the general existence of the structureless (limitary) membrane inclosing the cytoblasts, as the membrana propria of the true glands incloses the secreting cells (the spleen in which it is absent having perhaps its analogue in the liver among the true glands); (c) the arrangement

of the capillary vessels on the exterior of the vesicles or other collections of cytoblasts in the glands without, just as in those with, ducts. Thus, the glands without ducts possess all the apparatus which, in the true glands, is provided for secretion. They differ from them in that the formation of cells around their cytoblasts is exceptional, and that their secretion is poured into closed cavities, not into open canals.

Assuming then, that the common occupation of these organs consists in withdrawing from the blood some material which they may (probably after some elaboration) discharge into it again,-the next question is,-What is more particularly the function of each of these glands? In answer only one thing is proved; Mr. Simon's observations prove that in the hybernating animals the thymus forms in itself a store of fat, to be consumed in the maintenance of the temperature during hybernation; and this is more than ever yet was proved of any of these glands. His and former observations also render it highly probable that in other than the hybernant animals, the thymus, during all its temporary existence, is occupied in sequestrating some material from the blood to be restored to it again, in the same or (more probably) some other form. He believes that this material is always such as may be consumed in the service of respiration, "the thymus gland fulfilling its use as a sinking fund in the service of respiration." [But the evidence appears to me insufficient for this conclusion, or even opposed to it; for when in hybernation the gland performs this function, and performs it in the highest degree, it is temporarily adapted for it, not by temporary development, but, as all the analogies of the formation of fat in other cases show, by temporary degeneration. The formation of an extraordinary quantity of fat in any part expresses a defective nutrition of that part; and when the thymus of hybernants accumulates fat at the approach of their winter sleep, it is probably rather because some process in general nutrition to which it before ministered is ceasing, than because it is now about to discharge in an extraordinary degree its ordinary function. I should regard the fatty degeneration or fatty atrophy of the thymus at the approach of each winter-sleep as an annual recurrence of a process analogous to that atrophy by diminution or total removal of substance which takes place once for all in the animals in which the thymus is not persistent. In each case the atrophy is an indication that the necessity for the ordinary acts of the thymus has ceased; but in the hybernants it is for new circumstances made to minister to a new purpose, till, at the cessation of the winter-sleep, and the recommencement of new growth, it begins again to be truly developed, and to form the more highly organic azotized compounds which it may restore to the blood for the nutrition of the fresh-growing tissues.]

For the thyroid gland, Mr. Simon believes (chiefly on the evidence of its comparative anatomy), that it supplies, in its simplest state, a vascular diverticulum to the stream of the cerebral circulation, and that in its higher development, its secretion bears some essential relation to the nutrition of the brain, such that, for instance, while the brain is at rest it may be separating from the blood the same materials as the brain in action takes from the blood.

In like manner he holds the spleen to be as a diverticulum to the systematic circulation when the vessels are filled after taking food; and, by the secretion of the Malpighian corpuscles, an organ in which nutritive matter may be stored up till the system needs it. And, lastly, he thinks the renal capsules may have with the generative system some such relation in alternating secretion as he supposes the thyroid gland to have with the brain.

In the place of these several theories, Oesterlen, in a long discussion, enunciates but one, and that not a new one; namely, that the acts of the glands without ducts are the taking of fluid from the blood, from which as a cytoblastema their cytoblasts are formed; and that these, after their completed development, liquefy and restore to the blood a material more fitted for nutrition than that which it gave for them. [And indefinite and incomplete as this theory is, I must confess it appears to me to express all that can as yet be considered very probable in the general physiology of the glands without ducts.]

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