212 



THE THYMUS. 



by fine intra-lobular lamellrc of connective tissue into follicles (0'5-1'5 mm.). 

 These follicles make up the gland substance, and they are usually polygonal when 

 seen in a section. Each follicle consists of a cortical and a medullary part, and the 

 matrix or framework of both consists of a fine adenoid reticulum whose meshes 

 are filled with lymph-corpuscles " (Fig. 93, a).] Many of these corpuscles 

 exhibit various stages of disintegration. In the medulla are found the concentric 

 corpuscles of Hassall. ["They consist of a central granular part, around which are 

 disposed layers of flattened nucleated endothelial cells arranged concentrically. 

 When seen in a section they resemble the ' cell-nests ' of epithelioma (Fig. 93, I). 



<Y 



Fig. 92. 



Section of the thymus gland of a cat, showing 

 one complete lobule with an outer cortical 

 part, a centre, 6, and parts of adjoining 

 lobules , lymphoid tissue; c, blood- 

 vessels injected ; tZ, connective tissue. 



Fig. 93. 



Elements of the thymus ( x 300) 

 a, lymph-corpuscles ; b, con- 

 centric coi-puscle of Hassall. 



They have also been compared to similar bodies which occur in the prostate. 

 They are most numerous when the gland undergoes its retrograde metamorphosis."] 



Simon, His, and others described a convoluted blind canal, the " central canal," 

 as occurring within the gland, and on it the follicles were said to be placed. Other 

 observers, Jendrassik and Klein, either deny its existence or regard it merely as a 

 lymphatic or an artificial product. Numerous fine lymphatics penetrate into the 

 interior of the organ, and many are distributed over its surface, but their mode of 

 origin is unknown. [They seem to be channels through which the tymph-CQr- 

 puscles are conveyed away from the gland.] Numerous blood-vessels are also dis- 

 tributed to the septa and follicles (Fig 92, c). 



Chemical Composition. Besides gelatin, albumin, soda-albumin, there are 

 sugar and fat, leucin, xanthin, hypoxanthin, formic, acetic, butyric, and succinic 

 acids. Potash and phosphoric acid are more abundant in the ash than soda, 

 calcium, magnesium (? ammonium), chlorine, and sulphuric acid (v. Gorup- 

 Besanez). 



[Function. As long as it exists, it seems to perform the functions of a true 

 lymph-gland. This view is supported by the fact that in reptiles and amphibians, 

 which do not possess tymph-glands, the thymus remains as a permanently active 

 organ. That the thymus forms colourless corpuscles was first maintained by 

 Hewson, and confirmed by His and Jendrassik. Extirpation (Friedleben) gave few 



