602 



VETERINARY PHYSIOLOGY 



IV. From the Branchial Arches. 



7. Thymus. 



1. Development. — The thymus is formed as epithelial out- 

 growths from the branchial arches — in mammals from the 

 ventral side chiefly of the third and, to a lesser extent, from the 

 fourth cleft. 



2. Position. — In man and in most mammals it lies in the 

 thorax just in front of the heart. Islets of thymus tissue are 



Fig. 231. — Section of the Lobules of the Thymus to show the Lobules, 

 with Hassall's Corpuscles in the Central Part. 



frequently found in and around the thyreoid. In the guinea- 

 pig it is entirely in the neck. 



3. Structure — It is composed of two lobes, each made up 

 of a series of separate lobules surrounded by a fibrous capsule 

 and showing a denser cortical and a less dense medullary part. 

 It consists essentially of a network of epithelial cells, which, in 

 the medullary part, are here and there massed together to form 

 concentric agglomerations of cells, some in a state of degenera- 

 tion — the Hassall's Corpuscles. In the meshes of the network 

 are lymphocyte-like cells which are probably derived from 

 outside the gland, but which, according to some observers, are 

 formed from the epithelial cells (fig. 231). 



