THE THYMUS GLAND. 



413 



sends septa down between them for a short distance (Fig. 177). 

 On section each follicle is found to be composed of a cortical 

 and medullary portion. The medullary portion is often only 

 a cavity, and, as a rule, is found to connect, by a passage 

 through the cortex, with a general cavity in the centre of the 

 lobule. This latter cavity again connects with the spiral cen- 

 tral canal. The follicle is composed of reticular connective 

 tissue, in whose meshes are cells and the thymic juice. The 

 reticulum forms an adventitia for the blood-vessels. In the 

 cortical portion this reticular tissue is made up of small, nucle- 

 ated cells, with long, fine, branching processes. In the medul- 

 lary portion, when present, the reticular cells have large nuclei, 

 and their processes are 

 coarse and short. Within 

 the meshes of the structure 

 thus described are cells, 

 fat-globules, capillaries, 

 and a peculiar, transpar- 

 ent, acid, viscid, albumi- 

 nous fluid the thymic 

 juice. The cells are : 1, 

 lymph-corpuscles, which 

 exist in the greatest abund- 

 ance ; 2, large, granular, 

 nucleated cells of various 

 sizes many of these have 

 long processes, and they 

 help to form, partly by a 

 process of vacuolation (Watney), the concentric corpuscles ; 

 3, giant cells ; 4, the concentric corpiiscles of Hassall. These 

 last consist of one or more cells, around which are arranged 

 concentric layers of flat, epithelioid cells. This concentric en- 

 velope suggests the epithelial cylinders seen in carcinomatous 

 growths. One or two of these corpuscles may be enclosed in 

 another common envelope, thus forming a compound concen- 

 tric corpuscle. These corpuscles are strongly refractive, and 

 are readily stained with carmine. They lie near the arteries, 

 and have an intimate relation with them. According to Afan- 

 assien, indeed, they are developed from the endothelium of the 

 arterial wall. There is a vascular plexus about the follicles, 

 from which capillaries pass into the interior, forming a fine net- 



Fio. 177. Portion of the calf s thymus, after His : The 

 rings of the arterial branches (a) and venous branches (b) 

 with the capillary net-work (c) and the cavities of the 

 acini (d). Frey. 



