522 OF ABSORPTION AND SANGUIFICATION. 



in the lateral growth of branching off-shoots from this central tubular axis. In its mature 

 state, therefore, it consists of an assemblage of glandular follicles, which are surrounded by 

 a plexus of blood-vessels; and these follicles all communicate with the central reservoir, from 

 which, however, there is no outlet. The Lymphatics are large, and communicate directly 

 with the Vena Cava ; but their immediate connection with the cavity of the Thymus body 

 has not yet been demonstrated. The cavities of the follicles contain a fluid in which a 

 number of corpuscles are found, giving it a granular appearance. These corpuscles are, for 

 the most part, in the condition of nuclei; but fully developed cells are found among them, at 

 the period when the function of this body seems most active. The chemical nature of the 

 contents at this period, closely resembles that of the ordinary proteine-compounds. It has 

 been commonly stated, that the -Thymus attains its greatest development, in relation to the 

 rest of the body, during the latter part of foetal life ; and it has been considered as an organ 

 peculiarly connected with the embryonic condition. But this is a mistake; for the greatest 

 activity in the growth of this organ manifests itself in the Human infant, soon after birth; 

 and it is then, too, that its functional energy seems the greatest. This rapid state of growth, 

 however, soon subsides into one of less activity, which merely serves to keep up its propor- 

 tion to the rest of the body: and its increase usually ceases altogether at the age of about 

 two years. From that time, during a variable number of years, it remains stationary in 

 point of size; but, if the individual be adequately nourished, it gradually assumes the cha- 

 racter of a mass of fat, by the development of the corpuscles of its interior into fat-cells, which 

 secrete adipose matter from the blood. This change in its function is most remarkable in 

 hybernating Mammals; in which the development of the organ continues, even in an in- 

 creasing ratio, until the animal reaches adult age, when it includes a large quantity of fatty 

 matter. The same is the case, generally speaking, among Reptiles. It is an important fact 

 in the history of this organ, that it is not to be detected in Fishes; and does not appear to 

 exist, either in the tadpole state of the Batrachian reptiles, or in the Perennibranchiate group; 

 so that we may regard it as essentially connected with pulmonic respiration.* 



688. Various facts lead to the conclusion, that the function of the Thymus, 

 at the period of its highest development, is that of elaborating and storing up 

 nutritive materials, to supply the demand which is peculiarly active during the 

 early period of extra-uterine life. The elaborating 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 conveyed away by the lymph- 

 atics. The provision of a store of nutritive matter seems a most valuable one, 

 under the circumstances 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 nourish- 

 ment. As the demand becomes less energetic, and as the supplies 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 nutrition of the body, but for the re- 

 spiratory process, when this has to be carried on for long periods as in hy- 

 bernating Mammals and in Reptiles 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 Kid- 

 neys ; that is, as a diverticulum for the blood transmitted through the bron- 

 chial 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. 



089. The Thyroid Gland bears a general analogy to the Thymus; but its 

 vesicles are distinct from each other, and do not communicate with any com- 

 mon reservoir. They are surrounded, like the vesicles of the true glands, 

 with a minute capillary plexus; and in the 1 fluid they contain, numerous cor- 

 puscles are found suspended, Avhich 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 propor- 



* See Mr. Simon's admirable Prize Essay on the Thymus Gland. 



