THE LYMPHATIC SYSTEM. 



127 



FIG. 154. 



lobules ; these, in turn, are made up of groups of the primary alveoli 



or follicles. The latter closely resemble lymph-follicles in structure, 



being limited by a 



fibrous envelope giv- 

 ing off slender tra- 



beculse, which are 



soon lost in the deli- 

 cate reticulum of 



connective tissue 



pervading all parts 



of the follicles. The 



meshes of the re- 

 ticulum are occupied 



by numerous lym- 



phoid cells, among 



which many capillary 



blood - vessels run. 



The adenoid tissue 



of the peripheral 



zone, or cortex, of 



the follicles is more 



closely packed with 



cells than that occupying the centre, or medulla, in consequence of 



which variation the medulla appears 

 lighter than the denser cortex. 



Scattered throughout the follicles 



. FIG. 156. 



Section of human thymus body, showing general arrangement of 

 follicles : a, fibrous tissue enveloping lymphoid tissue and sending 

 septa (') between the follicles (b) ; d, interfollicular tissue, contain- 

 ing blood-vessels (c). 



FIG. 155. 



Portion of the periphery of one of the folli- 

 cles of the foregoing section, more highly 

 magnified : a, fibrous tissue ; b, lymphoid 

 tissue, containing numerous capillaries (c). 



Portion of the same follicle, showing corpuscles of 

 Hassall (a), which represent the original epithelial 

 constituents of the organ. 



round or oval bodies are seen, which vary greatly in number and size 

 (20-175 /*), usually stain but faintly, and present an irregularly concen- 



