334 



THE DUCTLESS GLANDS 



[CH. XXIII. 



lymphatic glands by their red colour. He divides them into (1) hcemal 

 glands, which are characterised by the fact that the sinuses contain 

 blood only. The spleen is in fact a large haemal gland; and (2) 

 hcemal lymphatic glands, in which the sinuses are filled by a mixture 

 of blood and lymph. 



The Thymus. 



This gland is a temporary organ ; it attains its greatest size early 

 after birth, and after the second year gradually diminishes, until in 





PIG. 312. Thymus of a calf, a, cortex of follicle ; 6, medulla ; c, interfollicular tissue. 

 Magnified about twelve times. (Watney.) 



adult life hardly a vestige remains. At its greatest development 

 is a long narrow body, situated in the front of the chest behind 

 the sternum and partly in the lower part of the neck. It is of a 

 reddish or greyish colour, and is distinctly 

 lobulated. 



The gland is surrounded by a fibrous cap- 

 sule, which sends in processes, forming trabe- 

 culae, that divide the gland into lobes, and 

 carry the blood- and lymph-vessels. The large 

 trabeculae branch into small ones, which divide 

 the lobes into lobules. The lobules are further 

 subdivided into follicles by fine connective- 

 tissue. A follicle is polyhedral in shape, and 

 consists of cortical and medullary portions, 

 both of which are composed of adenoid or 

 lymphoid tissue, but in the medullary portion 

 the matrix is coarser, and is not so filled up with lymphoid cor- 

 puscles as in the cortex. Scattered in the lymphoid tissue of the 

 medulla are the concentric corpuscles of Hassall (fig. 313), which 



FIG. 313. The reticulum of 

 the thymus. a, lymph 

 cells ; b, corpuscles of 

 Hassall. (Cadiat.) 



