THYMUS GLAND. 



1093 



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 i.s 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 surrounding 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 -,-^-g inch for the 

 largest, and -57*5^ for the smallest, probably a 

 correct average, for the generality is about 

 ^--jj.- inch. Mingled with these I have 

 found in the thynuis of a calf, as well as 

 in that of a young guinea-pig, a few larger 

 corpuscles, about double the size of the 

 former, 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 



* The contrast in this respect between the thy- 



strong r solution 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. 



mus and the thyroid is very instructive, in both the 

 limitary membrane forms closed cavities, which in 

 the one are chiefly filled with secretion, in the other 

 with nuclei, the accredited agents of secretory action. 

 * Whatever may be the nature of the fluid said 

 to lie contained in the thymic cavities in which the 

 nuclei float, it is too small in quantity, and too little 

 apparent, to make it necessary to take it into 

 account ; certainly it never collects after the man- 

 ner of a secretion. 



4 A 3 



