THE LYMPHATIC SYSTEM. 



127 



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- FIG. 154. 



ing off slender tra- 

 beculae, 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 



Section of human thymus body, showing general arrangement of 

 follicles : a, fibrous tissue enveloping lymphoid tissue and sending 

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

 ing blood-vessels (c). 



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. 



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 ,u), usually stain but faintly, and present an irregularly concen- 



