172 



HISTOLOGY. 



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 hymus there- 

 fore is considered a blood forming organ. In ordinary sections it resembles 

 a lymph gland, from which it may be distinguished by the presence of 

 thymic corpuscles [Hassall's corpuscles] in its medulla. These corpuscles 

 are found exclusively in the medulla of the thymus. They are rounded 

 bodies, at first few in number and small (12-20 /* in diameter), but they 

 increase rapidly in size (to a diameter of 180 ( ) and new ones are con- 

 stantly forming. At birth they are numerous, varying in size as shown 

 in Fig. 193. To produce them, the nucleus and protoplasm of a reticular 

 tissue cell (entodermal) are said to enlarge. The nucleus loses its staining 

 capacity by changes in its chromatin, and a layer of deeply staining hyaline 



Degenerated epithelial cells. 



Flat epithelial cells 

 Degenerated nucleus. 



*: 



FIG. 194. THYMIC CORPUSCLES, IN SECTION, FROM A MAN 23 YEARS OLD. X 360. 



substance develops in the protoplasm. This increases until it fills the entire 

 cell, often being arranged in concentric layers. The nucleus becomes 

 obliterated. Neighboring cells are concentrically compressed by the en- 

 largement of this structure, and by hyaline transformation they may be- 

 come a part of the corpuscle. The larger corpuscles are due to a fusion 

 of smaller ones, or to hyaline changes occurring simultaneously in a group 

 of cells. The central portion of a corpuscle may become calcified. Some- 

 times it is vacuolated, containing fat. The hyaline substance may respond 

 to mucous stains, but generally it does not; it has been considered similar 

 to the 'colloid' of the thyreoid gland. Leucocytes are said to become im- 

 bedded in the corpuscles or to enter them and assist in their disintegration. 

 Thymic corpuscles have been regarded as degenerative products of the en- 

 todermal epithelium; as concentric connective tissue masses; and as blood 

 vessels with thickened walls and obliterated cavities. Injections show 



