TIIYMUS GLAND. 291 



rating action probably corresponds with that, which is exerted by the 

 glands of the Absorbent system ; and the product, as in the preceding 

 cases, seems to be taken back into the circulation. The provision of a 

 store of nutritive matter seems a most valuable one, under the circum- 

 stances in which it is met with ; the waste being more rapid and variable 

 than in adults, and the supply not constant. Thus it has been noticed 

 that, in over-driven lambs, the thymus soon shrinks remarkably ; but 

 that it becomes as quickly distended again during rest and plentiful 

 nourishment. As the demand becomes less energetic, and as the sup- 

 plies furnished by other organs become more adequate to meet it, the 

 Thymus diminishes in size,. -and no longer performs the same function. 

 It then obviously serves to provide a store of material, not for the nutri- 

 tion of the body, but for the respiratory process, when this has to be 

 carried on for long periods (as in hybernating Mammals, and in Rep- 

 tiles), without a fresh supply of food. It is possible that the Thymus 

 gland may further stand in the same relation to the Lungs, as the Spleen 

 to the Liver, and the Supra-Renal capsules to the Kidneys ; that is, as 

 a diverticulum for the blood transmitted through the bronchial arteries 

 (which are the nutritive vessels of the Lungs), before the Lungs acquire 

 their full development in comparison with other organs, or when any 

 cause subsequently obstructs the circulation through their capillaries. 



513. The Thyroid Gland bears a general analogy to the Thymus ; 

 but its vesicles are distinct from each other, and do not communicate 

 with any common reservoir. They are surrounded, like the vesicles of 

 the true glands, with a minute capillary plexus ; and in the fluid they 

 contain, numerous corpuscles are found suspended, which appear to be 

 cell-nuclei, in a state of more or less advanced development. This body 

 is supplied with arteries of considerable size ; and with peculiarly large 

 lymphatics. Though proportionably larger in the foetus than in the 

 adult, it remains of considerable size during the whole of life. It ap- 

 pears, from the recent inquiries of Mr. Simon,* that a Thyroid gland,, 

 or some organ representing it in place and office, exists in all Verte- 

 brated animals. It presents its simplest form in the class of Fishes ; 

 in some of which it appears to consist merely of a plexus of capillary 

 vessels, connected with the origin of the cerebral vessels, and capable, 

 by its distensibility, of relieving the latter, in case of any obstruction 

 to the proper movement of blood through them. In the higher forms 

 of this organ, the glandular structure, consisting pf the closed vesicles 

 over which the capillary plexus is distributed, and of their cellular con- 

 tents, is superadded : and the organ then appears, like the Spleen, to 

 be destined for two different uses ; namely, to serve as a diverticulum 

 to the Cerebral circulation ; and to aid in the elaboration of nutritive 

 matter, which is probably taken up by the Absorbent system, to be 

 again poured by it into the general current of the circulation. 



514. Thus the Spleen, the Supra-Renal Capsules, the Thymus Gland, 

 and the Thyroid Gland, all seem to share in the preparation of the nu- 

 tritive materials of the blood; in fact, we may regard them all as toge- 

 ther constituting an elaborating apparatus, which is precisely analogous 



* Philosophical Transactions, 1844. 



