LVMl'llATIC URUAXS 181 



are known as medullary cords. In the medulla are found a number 

 of spherical or oval bodies composed of concentrically arranged epi- 

 thelial cells. These are known as IlassaWs corpuscles (Fig. io6), and 

 represent the only remains of the original glandular epithelium. 

 They are characteristic of the thymus. The central cells of the cor- 

 puscles are usually spherical and contain nuclei, while the peripheral 

 cells are flat and non-nucleated. As the entire corpuscle takes a 

 bright red stain with eosin-hasmatoxylin, the corpuscles stand out 

 sharply from the surrounding bluish or pinkish lymphatic tissue. 

 With low magnihcations they 

 are apt to be mistaken for blood- 

 vessels. 



Unlike ' the other lymphatic (t; 

 organs, the lymph nodules of 

 the thymus contain no germinal 



centres. Mitosis can, however, ■ 



usually be seen in the lymphoid '■< . 

 cells. No definite lymph sinuses 

 have been demonstrated. Nu- ^ , ^ 



Cleated red blood cells occur m j,^^_ io6.-Hassall's"Corpuscle and Small 



the thymus, which must there- Portion of Surrounding Tissue. X6oo 



. ' . , , ^ ,1 (Technic page 182.) 

 tore be considered one 01 the 



sources of red blood cells as well as of lymphoid cells. 



Blood-vessels. — The larger arteries run in the connective-tissue 

 septa. From these, smaller intralobular branches are given off, 

 which break up into capillary networks in the cortex and medulla. 

 The capillaries pass over into veins. These converge to form larger 

 veins, which accompany the arteries. 



Of the lymphatics of the thymus little is known. They appear 

 to originate in indefinite sinuses within the lymphoid tissue, whence 

 they pass to the septa where they accompany the blood-vessels. 



Nerves. — These are distributed mainly to the walls of the blood- 

 vessels. A few fine fibres, terminating freely in the lymphatic tissue 

 of the cortex and of the medulla, have been described. 



The thymus originates in the entoderm in the region of the third bran- 

 chial groove, first as two hollow evaginations of the endothelium of the 

 pharyngeal cavity, which later become solid cords, and then separate entirely 

 from the pharynx. The thymus thus begins its fcetal existence as a typical 

 epithehal gland. Into this epithelial structure mesodermic cells grow and 

 differentiate into lymphatic tissue. This almost completely replaces the epi- 

 thelial tissue, only rudiments of which remain as Hassall's corpuscles. 



