Mature structure of the gland. — The results of minute scrutiny into the structure of the fully-developed thymus, accord well with those arrived at by other modes of inquiry. Its surface, when freed from inveating areolar tissue, exhibits, though in a raTher coarser manner, the minutely-divided appearance so characteristic of the conglomerate glands, and this is especially evident when fat cells have formed in the interstices of the lobules, pat terning the surface over with a network of white streaks. In a thin section taken from the gland and prepared for the microscope, the outlines of the vesicular cavities are rea dily seen ; they are much larger than those of the salivary glands, and vary very much in size ; in a human foetus, at about the mid period, they averaged inch, in a calf about inch, in a young guinea-pig they varied from A- to inch. Their form is oval or spherical, their outline distinct for about two thirds of their circumference, but in the remaining part blended with adjacent ones, so that there is never seen any thing resembling a detached and closed vesicle. The sharp definition of the outline by a clear dark line, gives full assurance of the presence of an in vesting limitary or basement membrane ; this constitutes a general envelope, forming the boundary of each of the glandular cavities, surrounding, therefore, the whole mass, but no where prolonged into an efferent canal, through which the contents might escape. In structure it is truly homogeneous, that is, considered per se, but as it is closely invested by a very thin layer of areolar tissue on its exterior, it has sometimes a kind of fibrous, striated, as pect. In all respects it closely resembles the basement tissue of other glands, and, as in them, I have never been able to perceive in it anything corresponding to the germinal cen tres of Mr. Goodsir.
The contents of the thymic cavities next demand our attention. They consist almost entirely of corpuscles, very closely resembling (in fact identical with) the nuclei of glandular cells ; the only difference which the most careful scrutiny can detect between them, I believe, is this, that they present more nume rous nucleoli than the nuclei of gland cells usually do. I doubt, however, whether even this is constantly the case. Their form is, I think, for the most part spherical ; Mr. Simon speaks of them as generally flat and circular, but I have never observed one, if this be their real form, presenting its thin edge to the eye, as blood-discs frequently do. They vary a good deal as to the condition of their interior spots or nucleoli, some contain two or three, some as many as four or five, a few have one only, and some of the smaller none at all, but are filled with a dimly molecular substance. Their envelope is strong and well defined, as that of nuclei always is. The extreme variations in size of these corpuscles, according to Mr. Simon, are inch for the largest, and for the smallest, probably a correct average, for the generality is about s-4noo inch. Mingled with these I have found in the thymus of a calf, as well as in that of a young guinea-pig, a few larger corpuscles, about double the size of the fornier, of spherical form, filled either with granular matter alone, or containing also a nucleus or larger vesicular body. I am by no means inclined to regard these as cells formed upon the originally-existing nuclei of the cavities, but rather as expansions of the nuclei themselves, with formation of granular matter in their interior.
It is well worthy of remark, that in the fully developed organ, before any appearance of atrophy has taken place, no other contents than those now described are found in the glandular cavities. There is none of the abun dant granular material which forms so large a part of the epithelium of most glands, no dif fused oily matter, the nuclei aggregated to gether into dense masses seem to fill the ultimate vesicles completely, and there is no trace of any material which can justly be re garded as the product of secretion.* A strongisolution of bichloride of mercury, in deed, coagulates a small quantity of diffused plasma (probably the liquor sanguinis of blood remaining in the capillaries) which adheres irregularly round the neuclei, but its effect on the contents of the thymusis very different from that which it has on the albuminoid epithe lium of the true glands. This is a remarkable circumstance, and, as yet, has not, I think, been sufficiently attended to. If we en deavour to interpret it, it would seem to im ply that the thymus is not truly a secreting organ, that is, that it does not separate from the blood or elaborate any special product, or in fact any product at all ; but that its func tion is limited to the formation of an appa ratus, which conforming closely to the type of secretory glands is yet not endowed with any analogous property. * The centres around which the material of the secretion should be evolved are present in myriads, but no gran ular substance analogous to that of glandular epithelium is formed around them. It seems, therefore, that in the case of the thymus, the liquor sanguinis exuded from the vascular plexus through the homogeneous tunic simply solidifies into cytoblasts or nuclei, in most other glands a part takes the same organic form, —a certain number of nuclei are formed,— but these then become the centres of a dif ferent and more complete action, or are en dued with peculiar attractive powers, in virtue of which the materials of the several secretions collect around them in their respective labo ratories. Such is the fact microscopic inquiry adduces, and such the interpretation which may be offered:of it ; let us now turn to chemistry, and inquire whether the view we have just suggested is supported or negatived by the result of analysis. Mr. Simon gives three analyses of the thymus, which, as he states, though performed on the tissue itself, and not on its fluid contents, may fairly be depended on for conveying a sufficiently correct idea of the chemical constitution of the matters con tained in its cavities. Now in none of these is there any mention of any special substance which could be regarded as characterizing the secretion, on the contrary, the constituent elements are mere fibrinous, albuminous, or extractive matters and ordinary saline com pounds, and there is none of these which might not exist in the blood, and be most readily derived from it. This is, in fact, the con clusion which Mr. Simon adopts ; he believes that we may express the nature of the secre tion of the thymus as nearly as may be by the formula of Proteine, or denominate it, in physiological language, as simply nutrient matter.