388 HANDBOOK OF THl'SIOLOGY. 



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 trabeculaa 

 branch into small ones, which divide the lobes into lobules. The gland 

 is incased in a fold of the pleura. The lobules are further subdivided 

 into follicles by fine connective tissue. A follicle (Fig. 2G6) 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 

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

 nodal points, and the other inclosed within the first, composed of 

 branched connective-tissue corpuscles (Watney). Scattered in the ade- 

 noid tissue of the medulla are the concentric corpuscles of Hassall, which 

 are protoplasmic masses of various sizes, consisting of a nucleated gran- 

 ular centre, surrounded by flattened nucleated endothelial 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. 

 Haemoglobin is found in the thymus of all animals, either in these cysts, 

 or in cells near to or of the concentric corpuscles. In the lymph issuing 

 from the thymus are cells containing colored blood-corpuscles and haemo- 

 globin granules, and in the lymphatics of the thymus there are more 

 colorless cells than in the lymphatics of the neck. In the blood of the 

 thymic vein, there appears sometimes to be an increase in the colorless 

 corpuscles, and also masses of granular matter (corpuscles of Zimmer- 

 mann) (Watney). 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. 



Function. The thymus appears to take part in producing colored 

 corpuscles, both from the large corpuscles containing haemoglobin, and 

 also indirectly from the colorless corpuscles (Watney). 



Respecting the thymus gland in the hybernating animals, in which 

 it exists throughout life, as each successive period of hybernation ap- 

 proaches, the thymus greatly enlarges and becomes laden with fat, 

 which accumulates in it and in fat glands connected with it, in even 

 larger proportions than it does in the ordinary seats of adipose tissue. 

 Hence it appears to serve for the storing up of materials which, being 

 reabsorbed in inactivity of the hybernating period, may maintain the 



