290 ABSORPTION AND SANGUIFICATION. 



mals, however,, these bodies retain through life the same relative deve- 

 lopment. 



511. The Thymus Gland is another body, which seems referrible to 

 the same group ; having all the essential characters of a true gland 

 ( 714), save an excretory duct ; and its function being evidently con- 

 nected, during the early period of life at least, with the elaboration of 

 nutritive matter, which is to be reintroduced into the circulating cur- 

 rent. Its elementary structure may be best understood from the 

 simple form it presents, when it is first capable of being distinguished 

 in the embryo. It then consists of a single tube, closed at both ends, 

 and filled with granular matter ; and its subsequent development con- 

 sists in the lateral growth of branching oif-shoots from this central 

 tubular axis. In its mature state, therefore, it consists of an assem- 

 blage 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 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 proportion 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 character of a mass of fat, by the development of the cor- 

 puscles of its interior into fat-cells, which secrete adipose matter from 

 the blood. This change in its function is most remarkable in hyber- 

 nating Mammals ; in which the development of the organ continues, 

 even in an increasing ratio, until the animal reaches adult age, when it 

 includes a large quantity of fatty matter. The same is the case, gene- 

 rally 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 essen- 

 tially connected with pulmonic respiration.* 



512. 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 pecu- 

 liarly active during the early period of extra-uterine life. - The elabo- 



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



