484 



SECRETION". 



tion 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 simple sketch of its structure, as compared with the ductless glands already 

 considered. 



The thymus appears at about the third month of foetal life, and it gradually increases in 

 size until about the end of the second year. It then undergoes atrophy, and it disappears 

 almost 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 pericar- 

 dium, extending as low as the fourth costal cartilage. The cervical portion extends 

 upward as far as the lower border of the thyroid gland. The whole gland is about two 

 inches in length, one and a half inch broad at its lower portion, and about one-quarter 

 of an inch thick. Its color is grayish, with a slightly rosy tint. It is usually in the form of 

 two lateral lobes lying in apposition in the median line, although sometimes there exists 

 but a single lobe. It is composed of numerous lobules held together by fibrous tissue. 



The proper coat of the thymus is a delicate fibrous 

 membrane, sending processes into the interior of the 

 organ. Its fibrous structure, however, is loose, so that 

 the lobules can be separated with little difficulty. Por- 

 tions 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 arrange- 

 ment is more distinct in the inferior animals of large 



FIG. 143. Unravelled thymus from the calf; 



natural size. (Koll'iker.) 

 a, a, cord of the thymus ; &, ft, &, lobules : c, 

 small nodules attached to the cord. 



FIG. 144. 



-Half of the human thymus, laid open in 

 its lower portion. (Koiliker ) 



size than in man. The lobules are composed of rounded vesicles, from ten to fifteen in num- 

 ber, and from ^ to ^ of an inch in diameter. The walls of these vesicles are thin, 

 finely granular, and excessively fragile. The vesicles contain a small quantity of an 

 albuminoid fluid, with cells and free nuclei. The cells are small and transparent, and the 

 nuclei, spherical, relatively large, and containing from one to three nucleoli. The free 

 nuclei are also rounded and contain several distinct nucleoli. These vesicles are easily 





