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A TEXTBOOK OF PHYSIOLOGY 



and a pale medulla. All the lobules in each half of the thymus are 

 attached to a cord of medullary substance, as may be seen if the organ 

 is pulled apart. The thymus resembles in structure a lymphatic 

 gland, but germinal centres are absent, and there is nothing to cor- 

 respond with a lymph sinus. The cortex is crowded with lymphocytes, 

 and is very vascular. The medulla is more open in texture, and is 

 characterized by the presence of the concentric corpuscles of Hassall. 



c b k d e i <t 



FIG. 258. MESIAL SAGITTAL SECTION THROUGH THE PITUITARY BODY OF AN ADULT 

 MONKEY. (Herring, from Quarterly Journal of Experimental Physiology.) 



a, Optic chiasma ; b, tongue-like process of pars intermedia ; c, third ventricle ; 

 d, anterior lobe ; e, epithelial cleft of posterior lobe ; k, epithelium of pars inter- 

 media extending over and into adjacent brain substance. The dark shading 

 indicates anterior lobe proper ; the lighter shading shows the position of the 

 epithelium of pars intermedia ; </, nervous substance of posterior lobe ; i, epithelial 

 investment. 



These are generally regarded as degenerated products of entodermal 

 epithelium. Some authorities maintain that the thymic cells are not 

 true lymphocytes, but are of entodermal origin. 



The Function of the Thymus. By virtue of its lymphatic tissue, 

 the thymus gives origin to the lymphocytes of the blood, and possibly 



