THYMUS 



225 



small round epithelial cells deceptively similar to lymphocytes. Of true leucocytes 

 in the thymus he says, "In the places where the medulla is directly in contact with the 

 surrounding connective tissue and such places become constantly larger and more 

 numerous as the organ grows many leucocytes wander into the medulla; they lie 

 in the connective tissue surrounding the medulla but not in that around the cortex 

 (Fig. 214)." He considers that the cortex with its many mitotic figures represents a 

 zone of production, and the medulla, a zone of growth and degeneration (Anat. Hefte, 

 Abth. i, 1906, vol. 31, pp. 409-457). Hammar (1905, loc. cit.} is unable to determine 

 the source of the "thymus lymphocytes," but is confident that the reticulum is of 

 epithelial origin. He finds that in birds this reticulum produces cells resembling 

 striated muscle fibers, and these "myoid cells" he considers to be entodermal. In 

 his later work. (1911, loc. cit.} he states that the lymphocytes enter the thymus chiefly 

 from 'the thymic blood vessels. 



Not only lymphocytes, but other leucocytes, eosinophilic cells, and 

 multinuclear giant cells have been found in the medulla. Erythroblasts 

 are said to occur in its outer portion and in the cortex. The thymus 



Degenerated epithelial cells. 



Flat epithelial cells. 

 Degenerated nucleus. 



FIG. 215. THYMIC CORPUSCLES, IN SECTION, FROM A MAN TWENTY-THREE YEARS OLD. X 360. 



therefore is sometimes considered a blood-forming organ. Sometimes 

 the medulla contains cysts, which may be lined in part with typical 

 ciliated cells. The most characteristic structures in the thymus are the 

 thymic corpuscles (Hassall's corpuscles) which are found exclusively in 

 the medulla. They are rounded bodies, at first few in number and 

 small (i 2-20 [1 in diameter), but they increase rapidly in size (to a diameter 

 of 1 80 //) and new ones are constantly forming. They are said to be 

 present at about the fifth month, and at birth they are numerous, varying 

 in size as shown in Fig. 215. To produce them, the nucleus and pro- 

 toplasm of an entodermal reticular tissue cell enlarge, and the nucleus 

 loses its staining capacity by changes in its chromatin. A layer of 

 deeply staining hyaline substance develops in the protoplasm. This in- 

 creases until it fills the entire cell, often being arranged in concentric 

 layers, and the nucleus becomes obliterated. Neighboring cells are con- 

 centrically compressed by the enlargement of this structure, and by hyaline 

 is 



