1882.] 



The Minute Anatomy of the Thymus. 



351 



carmine and carmine, the network is stained of a different colour to' 

 the reticulum. 



The network of the cortex is continuous with a network in the 

 medulla ; in the latter situation, however, the cells are large and their 

 processes are coarse. There is a gradual transition between these last 

 and epithelioid connective-tissue corpuscles ; and, further, in some 

 places the network assumes the form of multi-nucleated protoplasmic 

 masses not differentiated into cells. In the medulla but few lymphoid 

 cells and only traces of the reticulum are seen, but it contains con- 

 centric corpuscles, giant cells, and numerous granular and epithelioid 

 cells. 



Granular cells are met with in the later period of embryonic life ; 

 they are not readily acted on by ordinary staining solutions ; their 

 protoplasm resembles that of the giant cells of the medulla of bone. 

 They are present in four forms : — (1) As polygonal or rounded 

 epithelioid cells, the central part only of the cells being granular ; (2) 

 as vacuolated cells, the mass lying in the vacuole being granular ; (3) as 

 spherical masses lying in cavities between the branching processes of 

 the connective- tissue corpuscles ; and (4) as rounded or club-shaped 

 masses attached (often by fibrillated extremities) to blood-vessels and 

 to newly formed connective-tissue. The first form arises from epithe- 

 lioid connective- tissue corpuscles, and gives origin to the second and 

 third varieties. The fourth variety forms fibrous tissue, and sometimes 

 forms blood-vessels : the cells of this class are very similar in ap- 

 pearance to certain cells (Bildungszellen) which have been described 

 by Ziegler and Tillmanns in pathological new formations. 



The giant cells arise in two ways, either from the fusion of several 

 granular cells, or from the branched protoplasmic network. 



The concentric corpuscles consist of a central mass and of a capsule ; 

 the central mass is at times found passing down the vessel-like pro- 

 longations, which are attached to the concentric corpuscles ; the 

 capsule is formed of epithelioid cells; these cells are anatomically 

 continuous with the branched connective-tissue corpuscles forming the 

 network ; the cells of the network around the concentric corpuscles 

 are larger than in other parts of the medulla. The concentric 

 corpuscles are attached to one another by long coarse threads which 

 have nuclei imbedded in them, or by bands of fibrous tissue. They are 

 finally transformed into bands of fibrous tissue containing vessels. 

 The central part of the concentric corpuscles is never penetrated by 

 injection, although the outer part of the capsule often contains 

 vessels, as the capsule of epithelioid cells in its growth may surround 

 a vessel. The smaller concentric corpuscles are composed of one or 

 more granular cells, surrounded by epithelioid connective-tissue cor- 

 puscles ; they arise from these two sources. 



Ciliated epithelium is found lining cysts in the thymus of the dog. 



