THE THYMUS BODY. 



20 1 



that by the middle of fcetal life the organ has acquired a lobulated structure, 

 a condition intensified by the ingrowth of vascular mesodermic tissue. 

 Meanwhile, the original closely packed epithelial elements undergo marked 

 change, most being converted into stellate cells that form a reticulum. 

 From other cells arise by repeated division a profusion of very small cells 

 that fill the meshes of the reticulum produced by the transformation just 

 mentioned. The genetic relation of the small cells to the original ento- 

 dermic epithelium is still disputed. According to Stohr, Bell and others, 

 they arise from the epithelial elements; according to Hammar and others, 

 they are mesodermic cells that early enter the thymus and correspond to 



FIG. 249. Section of developing thymus from human foetus of third month ; among the stellate reticulum- 

 cells are seen the small thymic lymphocytes. X 690. (Hammar.) 



true lymphocytes. All are agreed, however, that the "small cells" 

 the thymic lymphocytes closely resemble morphologically the ordinary 

 lymphocytes. 



The thymus is invested by a loose fibro-elastic capsule, from which 

 septa, rich in blood-vessels and lymphatics, pass inwards and subdivide the 

 organ into a number of indefinite lobes. The latter are broken up by partial 

 partitions into lobules, in which a denser peripheral tract, the cortex, and a 

 lighter central one, the -medulla, can be distinguished, although these divi- 

 sions are often not sharply defined. The thymus possesses no duct-system 

 and, hence, is often classed as a ' ' ductless gland. ' ' 



The cortex consists of closely packed small cells (7-10 /*. in diameter), 

 whose cytoplasm is so meagre that the deeply staining nuclei are their most 

 evident parts. The small cells are supported by a delicate meshwork formed 

 by the stellate reticulum- cells. Numerous capillary blood-vessels, with 

 accompanying scanty strands of fibrous tissue, are intermingled with the 

 cortical elements. The medulla is of looser texture and contains, in addi- 

 tion to the small and reticulum-cells, much larger epithelial cells, either 

 singly or in limited groups and cords. 



The most distinctive feature of the medulla and, indeed, of the entire 

 organ, which otherwise bears a general resemblance to lymphoid tissue is 

 the presence of the irregularly spherical or elongated thymic bodies, or 

 corpuscles of Hassall, which appear as small lighter areas scattered through- 

 out the medulla. They first appear about the middle of fcetal life and 



