THYMUS AND THYROID GLANDS. 565 



that these organs have proportionally the largest size during the development 

 of the Kidneys, when their secreting function can scarcely be supposed to 

 have commenced. 



X. Thymus and Thyroid Glands. 



711. The Thymus Gland, also, is largest in the Foetus, in proportion to the 

 size of the body ; but it continues to grow after birth, and remains of considera- 

 ble size during the first year ; after which it remains stationary, or gradually 

 diminishes. It seldom, however, entirely disappears, as is usually stated ; and 

 several cases are mentioned by Krause, in which it remained in the adult of 

 the full size it possessed at birth : but, as the rest of the body has undergone 

 an increase to the amount of twenty times its original weight, the Thymus 

 may be said to have relatively diminished to the same amount. According to 

 the accurate examination of its structure, recently made by Sir A. Cooper, it 

 is composed of lobules, which may be drawn out and separated from one ano- 

 ther in the manner of a string of beads, when their enveloping capsule has 

 been removed. These lobules vary in size from that of the head of a pin to 

 that of a pea; in their usual position, they are disposed around a large central 

 cavity or reservoir. When a thin slice is cut from one of the lobules, a num- 

 ber of small cavities are seen in it ; and these are filled with a white granular 

 fluid. These cavities open into a larger one at the base of each lobule ; and 

 the cavities of the different lobules are connected by a channel, which passes 

 from one to the other, and which has occasional openings leading into a com- 

 mon reservoir. This reservoir, however, has no efferent duct ; and no means 

 of exit can be detected for its contents, except that afforded by the Lymphatics, 

 which are large, and terminate directly in the Vena Cava; their immediate 

 connection with the cavities of the gland has not, however, been demonstrated. 

 The fluid of the Thymus Gland is whitish, having the appearance of chyle or 

 cream ; it contains a large number of corpuscles, which are described by Mr. 

 Wilson,* as being " smaller than the blood-corpuscles, globular and oval in 

 form, irregular in outline, variable in size, and provided with a small central 



Fig. 179. 



A section of the Tliymus gland at the eighth month, showing its anatomy; from a preparation of Sir A, 

 Cooper's; 1, the cervical portions of the gland ; the independence of the two lateral glands is well marked; 

 2 secretory follicles seen upon the surface of the section; these are observed in all parts of the section; 

 3, 3, the pores or openings of the secretory follicles and pouches ; they are seen covering the whole internal 

 surface of the great central cavity or reservoir. The continuity of the reservoir in the lower or thoracic 

 portion of the gland with the cervical portion, is seen in the figure. 



* Anatomist's Vade-Mecum .p. 526. 



48 



