CH. XXIII.] 



THE THYMUS. 



329 



Scattered in the lymphoid tissue of the medulla are the concentric 

 corpuscles of Hassall (fig. 304), which consist of a nucleated 

 granular centre, surrounded by flattened nucleated epithelial 



Fig. 302. Thymus of a calf, a, cortex of follicle; b, medulla; c, interfoUicular tissue. 

 Magnified about twelve times. (Watney.) 



cells. These are islands of epithelial cells cut off from the 

 epithelium of the pharynx in process of development. They are 

 not occluded blood-vessels, as was at one time supposed. They 



' 



Fig. 303. From a horizontal 

 section through superfi- 

 cial part of the thymus 

 of a calf, slightly magni- 

 fied. Showing in the 

 centre a follicle of poly- 

 gonal shape with -imi- 

 larly shaped follicles 

 round it (Klein and 

 Noble Smith.) 



r-v 



Fig. 304. The reticulum of the 

 tnymus. a, lymph cells ; 

 b, corpuscles of Hassall. 

 (Cadiat.) 



remind one somewhat of the epithelial nests seen in some 

 varieties of cancer. 



The arteries radiate from the centre of the gland. Lymph sinuses 

 may be seen occasionally surrounding the periphery of the follicles 

 (Klein). The nerves are very minute. 



