204 NORMAL HISTOLOGY. 



creasingly more voluminous, with separation of and encroachment upon the 

 diminishing thymic tissue. Gradually the latter is replaced by the adipose 

 tissue, until only isolated islands of the characteristic thymus-tissue remain 

 (Fig. 252). Complete disappearance of this structure, however, is very 

 exceptional, even in advanced age a certain amount of it being recognizable 

 upon microscopical examination. 



The blood-vessels distributed to the thymus send their twigs into the 

 organ in such manner, that these lie at the junction of the cortex and medulla 

 (Fig. 250). Capillaries thence proceed to the cortex and medulla, those 

 within the former being more abundant than those distributed to the medulla. 

 The venous radicles begin in the thymus-stroma, some passing through the 

 medulla while others become more directly tributaries of the interlobular 

 veins. The lymphatics are numerous and represented by networks of 

 lymph-spaces close to the periphery of the lobules, from which are formed 

 the more definite interlobular lymphatic vessels, that in turn drain into the 

 large efferent trunks. The existence of intralobular passages, corresponding 

 to lymph-sinuses, has not been established. The nerves are small and are 

 derived from the sympathetic and the vagus. The fibres are traceable along 

 the interlobular septa, in company with the blood-vessels, to whose walls 

 they are chiefly distributed. A very meagre number of nonmedullated fibres 

 have been described as terminating within the medulla as free endings. 



