324 



HANDBOOK OF PHYSIOLOGY. 



The Thymus. This gland must be looked upon as a temporary 

 organ, as it attains its greatest size early after birth, and after the 

 second year gradually diminishes, until in adult life hardly a vestige 

 remains. At its greatest development it is a long, narrow body, situated 

 in the front of the chest behind the sternum and partly in the lower part 

 of the neck. It is of a reddish or grayish color, distinctly lobulated. 



Structure. The gland is surrounded by a fibrous capsule, which sends 

 in processes, forming trabeculae, which divide the glands into lobes, and 

 carry the blood and lymph-vessels. The large trabeculae branch into 

 small ones, which divide the lobes into lobules. The lobules are further 



Fig. 232. 



Fig. 833. 



Fig. 232.- From a horizontal section through superficial part of the thymus of a calf, slightly 

 magnified. Showing in the centre a follicle of polygonal shape with similarly shaped follicles 

 round it. (Klein and Noble Smith.) 



Fig. 233. The reticulum of the Thymus. a, Epithelial elements; &, corpuscles of Hassall. 

 (Cadiat.) 



subdivided into follicles by fine connective tissue. A follicle (fig. 232) 

 is seen on section to be more or less polyhedral in shape, and consists of 

 cortical and medullary portions, both of which are composed of adenoid 

 tissue, but in the medullary portion the matrix is coarser, and is not so 

 filled up with lymphoid corpuscles as in the cortex. The adenoid tissue 

 of the cortex, and to a less marked extent that of the medulla, consists 

 of the two elements, one with small meshes formed of fine fibres with 

 thickened nodal points, and the other enclosed within the first, com- 

 posed of branched connective-tissue corpuscles (Watney) . Scattered in 

 the adenoid tissue of the medulla are the concentric corpuscles of Hassall, 

 which are protoplasmic masses of various sizes, consisting of a nucleated 

 granular centre, surrounded by flattened nucleated epithelial cells. 

 In the reticulum, especially of the medulla, are large transparent giant 

 cells. In the thymus of the dog and of other animals are to be found 

 cysts, probably derived from the concentric corpuscles, some of which 

 are lined with ciliated epithelium, and others with short columnar cells. 

 The arteries radiate from the centre of the gland. Lymph sinuses may 

 be seen occasionally surrounding a greater or smaller portion of the 

 periphery of the follicles (Klein). The nerves are very minute. 



