THE THYMUS 



159 



FIG. 155. A CORPUSCLE OF HASSAL FROM THE THYMUS 



OF AN INFANT. 



Hematein and eosin. x 665. 



except for the fact that the meshes of the reticulum in the cortex 

 are much more crowded with leucocytes than are those of the 

 medulla. The medulla of each lobule is also characterized by 

 the presence of several 

 groups of concentrical- 

 ly arranged epithelioid 

 cells, the concentric cor- 

 puscles of Hassal. 



Each concentric cor- 

 puscle consists of a 

 large central cell or 

 group of cells, which 

 is surrounded by two 

 to five layers of con- 

 centrically arranged flat 

 epithelioid cells. These 

 groups or cell nests are 

 strongly acidophile in 

 their staining reactions, 

 and therefore stand out 

 in marked contrast to the basophilic nuclei of the surrounding 

 lymphoid tissue. Since no similar structure occurs elsewhere in 

 the body, the concentric corpuscles are absolutely characteristic 

 of the thymus lobule. 



The nature of the concentric corpuscles is not satisfactorily 

 understood. According to one hypothesis they represent blood 

 vessels whose lumen has been obliterated by proliferation of its 

 endothelial cells. Another theory regards them as remains of the 

 epithelial columns from which the organ arises in the embryo. 



The types of lymphatic corpuscle which are found in the thymus 

 are similar to those of the lymphatic glands, though polynuclear 

 leucocytes are rather more frequent here, and giant cells, polynu- 

 clear or multinuclear in form, may be readily found in the medulla 

 of this organ. Eosinophile cells have been found in the thymus 

 even at a very early period of embryonic life (Schaffer*), and 

 Beard f has ascribed the first formation of embryonic leucocytes 

 to this organ. 



Blood supply. The larger arteries of the thymus are distributed 

 within the interlobular connective tissue. They supply branches 



* Centralbl. f. d. raed. Wissensch., 1891. 

 f Lancet, 1899 ; and Anat. Anz., 1900. 



