362 8ECKETION. 



the situation previously occupied by this gland. As there 

 never has been a plausible theory, even, of the function of 

 this organ, the existence of which is confined to the first two 

 or three years of life, we shall abstain from all discussions 

 with regard to minute points in its anatomy, and give a sim- 

 ple sketch of its structure, as compared with the ductless 

 glands already considered. 



The thymus appears about the third month of foetal life, 

 and gradually increases in size until about the end of the 

 second year. It then undergoes atrophy, and disappears al- 

 most entirely at the age of puberty. It is situated partly in 

 the thorax and partly in the neck. The thoracic portion is 

 in the anterior mediastinum, resting upon the pericardium, 

 extending as low as the fourth costal cartilage. The cervical 

 portion extends upward as far as the lower border of the 

 thyroid. The whole gland is about two inches in length, 

 one and a half inches broad at its lower portion, and about 

 one-quarter of an inch thick. Its color is grayish, with a 

 slight rosy tint. It is usually in the form of two lateral 

 lobes, lying in apposition in the median line, though some- 

 times there exists but a single lobe. It is composed of nu- 

 merous lobules, held together by fibrous tissue. 



The proper coat of the thymus is a delicate fibrous mem- 

 brane, sending processes into the interior of the organ. Its 

 fibrous structure, however, is loose, so that the lobules can 

 be separated with little difficulty. Portions of the gland 

 may be, as it were, unravelled, by loosening the interstitial 

 fibrous tissue. In this way it will be found to be composed 

 of numerous little lobular masses, attached to a continuous 

 cord. This arrangement is more distinct in the inferior ani- 

 mals of large size than in man. The lobules are composed 

 of rounded vesicles, from ten to fifteen in number, and from 

 T2T ^0 iV f an mcn i n diameter. The walls of these vesicles 

 are thin, finely granular, and excessively fragile. The vesi- 

 cles contain a small quantity of an albuminoid fluid, with 

 cells and free nuclei. The cells are small and transparent, 



