26 TISSUES AND ORGANS 



this organ that a good purpose should be served by going beyond 

 our terms of reference. 



It is particularly the small cells of the thymus that present 

 problems of difficulty and importance. These are small elements 

 with scanty cytoplasm and dense nuclei lying in the meshes of the 

 reticulum of the organ. Although these cells have been supposed 

 to be epithelial elements, there can be no doubt that their mito- 

 chondria are identical in appearance with those of ordinary 

 lymphocytes, and are quite different and much larger than those 

 of the reticular cells. The last-named are almost certainly the 

 original epithelial cells of the organ. This supporting structure is 

 rather hidden by the multitude of lymphocytes, but after X-irradia- 

 tion it is well seen, and shows evidence of great activity. The 

 cells rapidly divide, and many multi-nucleated giant cells are 

 formed ; both these and the cells of the reticulum are very 

 actively phagocytic. Even in the normal thymus the products of 

 the breakdown of the small cells are ingested by elements belonging 

 to the reticulum. 



It is the so-called HassaU's corpuscles that have attracted most 

 attention, because in no other organ are similar structures found. 

 The generally accepted modern view is that these corpuscles arise 

 from the reticulum of the organ, probably from hypertrophic and 

 degenerate elements. In man they are not seen until the second 

 month of foetal life ; they then continue to increase until age 

 involution of the organ commences, but it is quite probable that 

 new ones are formed in the thymic remnants throughout life. A 

 corpuscle may consist of a single reticular cell or of groups of the 

 same elements. The appearance of the structure depends upon 

 its age ; when young the constituent cells are irregularly polygonal, 

 but when old they are flattened and hyaline crescentic elements 

 arranged around a central part composed of debris. Calcification 

 or cyst formation is quite common. The latter condition, which 

 was described in syphilitic infants, is not a pathological state, but 

 may be seen in almost any thymus. 



A most interesting constituent of the thymus are the myoid 

 cells, which are fairly common in lower animals, although not 

 often described in the human. There is but little doubt that, in 



