DIGESTIVE ORGANS AND THEIR ANATOMY. 305 



three inches in width. The gland figures in the business 

 of the meat dealer, and is sold under the name of sweet- 

 breads. 



The thymus is made up of two lobes of almost the same 

 size, which lobes, however, usually lie close together, meet- 

 ing each other at their inner surfaces. In structure the 

 thymus gland is a typical lymphatic gland. It is invested 

 with a capsule of fibrous connective tissue, from which 

 trabeculse extend into the gland, subdividing it and form- 

 ing a framework throughout its interior. In the meshes of 

 this framework are imbedded innumerable white corpuscles. 

 The trabeculae extending in from the capsule divide the in- 

 terior off into little chambers or lobules in which the cor- 

 puscles show an arrangement in two layers, an outer layer 

 of ordinary white corpuscles closely packed, and similar in 

 every way to lymphoid structure wherever found. The 

 fact that these cells are spoken of as the thymus corpuscles 

 must not lead to the idea that they are in any way different 

 from ordinary lymphoid corpuscles. In the center of each 

 lobule the corpuscles are not arranged so closely, and there 

 are found here and there imbedded among the ordinary cor- 

 puscles nests of cells which show a concentric structure. 

 These are called the concentric corpuscles of Hassall. 

 Each nest seems to be composed of a covering of hardened 

 epithelium cells enclosing one or more granular cells. 

 While the meaning of these concentric corpuscles is not 

 wholly clear, there is much reason to believe that they are 

 but remains of a primitive epithelial tube which occurs in a 

 developing thymus and so have no physiological signifi- 

 cance. 



From its structure it would seem, then, that the thymus 

 gland is but a gigantic lymphatic nodule and as such is con- 

 cerned in the production of new white corpuscles. That it 

 disappears in advancing life may be easily accounted for by 

 the fact that new lymphatic nodules appear in many other 

 parts of the body which may relieve the thymus from the 

 necessity of further use. That the thymus reaches such a 

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