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THYMUS GLAND. 



and it cannot be deemed unreasonable to sup- 

 pose that its use in the system should change 

 also. In fact, that its use should be the same 

 in the two cases, where its functions are so 

 different, appears to us a very improbable 

 supposition." Consonant with the above 

 view is that of Mr. Paget, who remarks in 

 his Report, that the fatty transformation of 

 the thymus is an atrophy or degeneration of 

 the gland, " analogous to that atrophy by di- 

 minution 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 ne- 

 cessity for the ordinary acts of the thymus 

 has ceased ; but in the hibernants it is, for 

 new circumstances, made to minister to a new 

 purpose, till at the expiration of the winter 

 sleep, and the recommencement of new growth 

 it begins again to be truly developed, and to 

 form the more highly azotized organic com- 

 pounds which it may restore to the blood for 

 the nutrition of the fresh growing tissues." 



On a careful consideration of the theories 

 now noticed (and no other are worthy of any 

 examination), it seems to me, certainly, that 

 the latter is nearest the truth ; and that we 

 cannot regard the thymus in the young animal 

 as a mere preparer of material fitted to sup- 

 port the respiration. The following argu- 

 ments must be allowed to weigh strongly 

 against Mr. Simon's view. (1.) The chemi- 

 cal constitution of the thymus, consisting 

 chiefly of proteine and not of fatty matters, 

 seems by no means to be such as would be 

 best adapted to sustain the supply of com- 

 bustible material required in respiration. (2.) 

 The office of serving as a reservoir for ma- 

 terial to be used in respiration may be attri- 

 buted with much more probability to the 

 liver*, the construction of which, as also in 

 some measure its position relative to the cir- 

 culatory system, point it out as peculiarly 

 adapted for the reception of superfluous re- 

 spirable material from the blood, and equally 

 so for rendering it up again, when the demand 

 again begins to be felt in the circulating cur- 

 rent. Of this, the condition of the liver in 

 fish, and generally in all animals, in whom the 

 respiration is of a low type, is sufficient 

 proof. Now as the liver is of greater relative 

 size in the young than in the adult animal, it 

 is not likely that this one of its functions is 

 in any measure discharged by the thymus. 

 (3.) The anatomical constitution of the thymus 

 is very unlike that of an organ wliich serves 



* I may just briefly mention here the results to 

 which I have been led by a long and careful study 

 of the liver in vertebrata. Its lobules (if it is so 

 divided) are not penetrated by excretory ducts, nor 

 are the secreting cells contained as in most other 

 glands in the cavities of ducts, they lie naked in the 

 interstices of a close plexus of most delicate- walled, 

 capacious, capillaries, from which they readily re- 

 ceive the materials they are intended to act on ; and 

 to which they as readily render up their elaborated 

 products when these are called for. The excretory 

 ducts, consisting at their origins almost solely of 

 nuclei, attract into their cavities the oleo-biliary 

 secretion with which they are bathed; probably, 

 however, in so doing altering it to some extent. 



as a reservoir of respirable matter. For 

 this purpose we should expect to find a 

 fluid stored up in cavities, such as the oil of 

 the fat vesicles, or the oleo-biliary matter which 

 collects within the cells of the liver ; the ana- 

 tomical elements, however, of the thymus are 

 totally different, being mere nuclei, with no 

 trace even of commencing cell development, 

 in fact, just that part of a glandular appa- 

 ratus which is not the secretion, however im- 

 portant a part it may play in the elaboration 

 of it. (4.) The transformation of the thymus 

 into fat previous to the winter sleep, con- 

 currently with the formation of other fat 

 masses in the axillae, indicates, as above shown, 

 very strongly that the gland in its natural 

 state fulfils some purpose different from that 

 to which it is subservient during hibernation. 

 But as there can be no doubt that it undergoes 

 this change for the sake of the respiratory 

 function, to fit itself for supplying those de- 

 mands during the long period when the waste 

 of the tissues is reduced to a minimum, and 

 no ingesta are taken, it seems tolerably cer- 

 tain that while the gland continues in its 

 normal unchanged state, its action must be of 

 a different kind, not having special reference to 

 the function to which during hibernation it un- 

 doubtedly ministers. The circumstance quoted 

 by Mr. Simon from the writings of Mr. Gulliver, 

 respecting the remarkable shrinking of the 

 thymus in over-driven lambs, cannot be re- 

 garded as proving that the contents of its cavi- 

 ties are thus absorbed solely for the use of re- 

 spiration. It is more probable that they are 

 resumed into the blood, which has begun to be 

 impoverished by fasting and exercise, in order 

 that they may go to supply the nutrition of all 

 parts indifferently. With regard to the argu- 

 ments adduced by Mr. Simon from Compara- 

 tive Anatomy, to the effect that a thymus has 

 some essential connection with pulmonary 

 organs of respiration, I would remark that 

 though it is certainly of weight, yet it can- 

 not be regarded as proving absolutely that 

 the two organs are co-related in function. 

 It may be that they are only simultaneously 

 developed, connected together in virtue of 

 some law of organized being which requires 

 their coeval appearance, and yet not intended 

 to minister to a common pupose.* I grant 

 that observing their consentaneous appear- 

 ance, one should inquire whether they be not 

 essentially linked together in function, but if 

 this be not proved by subsequent inquiry, or 

 a fortiori, if it be shown to be unlikely, then 

 the argument arising from their co-develop- 

 ment ceases to have much force. 



But if we decline accepting the theory pro- 

 posed by Mr. Simon, are we to close, uncon- 

 ditionally, with the other which has been 

 above expounded, and which is but a modifi- 



* This is perhaps the more probable, seeing that 

 the thymus does not assert its connection with 

 organs of respiration taken generally, but only with 

 a particular modification of them, so that its exist- 

 ence is determined not so much by the degree in 

 which the function is fulfilled as by the mode. Hence 

 it is absent in the higher fishes and in insects. 



