I2 8 NORMAL HISTOLOGY. 



trie striation, with occasional nuclei ; these bodies are the corpuscles 

 of Hassall, or the concentric corpuscles. They represent the re- 

 mains of the epithelial structures which, as already stated, in the early 

 stages of the thymus constitute the principal tissue of the organ. 



The larger blood-vessels of the thymus run within the inter- 

 lobular connective tissue, giving off branches which penetrate the 

 follicles and break up into a rich capillary net-work supplying the 

 adenoid tissue of cortex and medulla. 



As may be inferred from the character of the organ, the lym- 

 phatics occur in large numbers. The radicles coming directly from 

 the follicles are received by the interlobular vessels, which, in turn, 

 communicate with the superficial net-work occupying the surface of 

 the organ. 



Bundles of nerve -fibres accompany the ramifications of the 

 arteries and veins, to the coats of which they seem principally to be 

 distributed. 



The thymus body reaches its highest development about the 

 second year, after which time it gradually diminishes, undergoing 

 retrogressive changes and absorption, until, by the eighteenth to the 

 twenty-first year, the characteristic tissues have disappeared or have 

 been replaced by fibrous connective tissue and fat. 



THE SEROUS MEMBRANES. 



The serous membranes are intimately related to the lymphatic 

 system, since the cavities which they enclose form parts of the gen- 

 eral lymph-tract of the body; when considered in their widest sig- 

 nificance they include the lining of all cavities clothed with endothe- 

 lial cells and cut off from atmosphere. Regarded in a more limited 

 and critical sense, such cavities may be separated into certain groups, 

 following which the connective-tissue linjngs may be divided into : 



a. The serous membranes proper, as the peritoneum, the pleura, 

 and the pericardium. 



b. The synovial membranes, including the synovial capsules of 

 the joints, the synovial sheaths of tendon, and the synovial bursse 

 placed between opposed movable surfaces to reduce friction. 



c. The endothelial lining of the vascular system, comprising that 

 of the heart, of the blood-vessels, and of the lymphatics. 



d. The lining of various spaces developed within the connective 

 tissues ; such spaces are usually small and provided with very rudi- 

 mentary linings ; they may be, however, of considerable size, as in 

 the case of the perilymphatic spaces of the internal ear. 



The serous membranes proper, represented by the peritoneum, 

 the pleura, the pericardium, and the tunica vaginalis, are all derived 

 as constrictions from the originally single pleuro-peritoneal cavity 



