1274 



SPLANCHNOLOGY 



appearance, and are aggregated to form lymphoid follicles. These lymphoid 

 cells are probably derivatives of the entodermal cells which lined the original 

 diverticula and their subdivisions. Additional portions of thymus tissue are 

 sometimes developed from the fourth branchial pouches. Thymus continues 

 to grow until the time of puberty and then begins to atrophy. 





Artery 



Vein 



Artery 



FIG. 1179. Minute structure of thymus. Follicle of injected thymus from calf, four days old, slightly diagram- 

 matic, magnified about 50 diameters. The large vessels are disposed in two rings, one of which surrounds the follicle, 

 the other lies just within the margin of the medulla. (Watney.) A and B. From thymus of camel, examined without 

 addition of any reagent. Magnified about 400 diameters. A. Large colorless cell, containing small oval masses of 

 hemoglobin. Similar cells are found in the lymph glands, spleen, and medulla of bone. B. Colored blood corpuscles. 



Structure. Each lateral lobe is composed of numerous lobules held together by delicate 

 areolar tissue; the entire gland being enclosed in an investing capsule of a similar but denser 

 structure. The primary lobules vary in size from that of a pin's head to that of a small pea, and 

 are made up of a number of small nodules or follicles, which are irregular in shape and are more 

 or less fused together, especially toward the interior of the gland. Each follicle is from 1 to 2 mm. 

 in diameter and consists of a medullary and a cortical portion, and these differ in many essential 

 particulars from each other. The cortical portion is mainly composed of lymphoid cells, supported 

 by a network of finely branched cells, which is continuous with a similar network in the medullary 

 portion. This network forms an adventitia to the bloodvessels. In the medullary portion the 

 reticulum is coarser than in the cortex, the lymphoid cells are relatively fewer in number, and 

 there are found peculiar nest-like bodies, the concentric corpuscles of Hassall. These concentric 

 corpuscles are composed of a central mass, consisting of one or more granular cells, and of a 

 capsule which is formed of epithelioid cells (Fig. 117,3). They are the remains of the epithelial 

 tubes which grow out from the third branchial pouches of the embryo to form the thymus. 



Each follicle is surrounded by a vascular plexus, from which vessels pass into the interior, 

 and radiate from the periphery toward the center, forming a second zone just within the margin 

 of the medullary portion. In the center of the medullary portion there are very few vessels, and 

 they are of minute size. 



Watney has made the important observation that hemoglobin is found in the thymus, either 

 in cysts or in cells situated near to, or forming part of, the concentric corpuscles. This hemo- 

 globin occurs as granules or as circular masses exactly resembling colored blood corpuscles. He 



