6 HAND-BOOK OF PHYSIOLOGY. 



sends in processes, forming trabeculse, which divide the gland into lobes, 

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

 small ones, which divide the lobes into lobules. The gland is encased in 

 a fold of the pleura. The lobules are further subdivided into follicles by 

 fine connective tissue. A follicle (Fig. 256) is more or less polyhedral in 

 shape, and consists of cortical and medullary portions, the structure of 

 both being 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 ex- 

 tent in the medulla, consists of two kinds of tissue, 

 one with small meshes formed of fine fibres with 

 thickened nodal points, and the other enclosed within 

 the first, composed of branched connective-tissue cor- 

 Fio.256. Fromahor- puscles (Watney). Scattered in the adenoid tissue of 



izontal section through ,-, in J.T / T ^ TT ^^ 



superficial part of the the medulla are the concentric corpuscles of Hassall, 



thymus of a calf, slight- -\ -\ i _c 



ly magnified. showing which are protoplasmic masses of various sizes, con- 

 sisting of a central nucleated granular centre, sur- 



rounded 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 found cells containing colored 

 blood-corpuscles and haemoglobin 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 in- 

 crease in the colorless corpuscles and also masses of granular matter (cor- 

 puscles of Zimmermann) (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). Eespecting the 

 function of the gland in the hybernating animals, in which it exists 

 throughout life; as each successive period of hibernation approaches, 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 re-absorbed in 

 inactivity of the hibernating period, may maintain the respiration and 



