THE THYMUS 329 



a single-lobed body, which comes to lie in the anterior media- 

 stinum in close relationship with the pericardium. 



B. Structure of the Thymus 



The thymus is made up of several lobules, which vary in 

 size, and are separated from one another by connective- 

 tissue septa, bearing bloodvessels and lymphatics. 



Each lobule may be divided into a cortical and a medullary 

 portion. The cortex is incompletely separated into " nodules " 

 by connective-tissue trabeculse, the arrangement bearing a 

 strong resemblance to that of a lymphatic gland. The cortex 

 is very vascular, and is similar in appearance to a lymphatic 

 gland. Its structure also agrees with that of lymph glands 

 and tonsils in exhibiting numerous signs of mitosis, but without 

 definite germ centres. In addition to the lymph cells, there 

 are also a number of peculiar granular cells. 



The medulla, like that of a lymphatic gland, is more open 

 in its texture than the cortex, and its reticulum is made up 

 of large, transparent, branched cells, which are sometimes 

 arranged in an epithelioid manner. The medulla does not 

 contain so many leucocytes as the cortex, but is characterized 

 by the presence of the peculiar concentrically striated bodies 

 the concentric corpuscles of Hassall (see Fig. 91). These vary 

 very considerably in general appearance, and their precise 

 origin and significance are still matters for discussion. 



The above account is largely derived from that given by 

 Schafer. It seems possible that there are many points in 

 connection with the thymus upon which current views may 

 have to be changed. It has long been taught that the human 

 thymus reaches its greatest development at about the second 

 year, and then begins to degenerate. But it was shown in 

 the year 1890 by Waldeyer that even in advanced age a con- 

 siderable amount of thymus tissue persists, and probably 

 maintains its function. Zoja had previously shown that the 

 thymus frequently persists till the age of puberty. Recently 

 Hammar has insisted that the organ continues to grow up to 

 the period of puberty, and reaches its greatest development 

 between the fourteenth and sixteenth years. From that time 

 onwards it gradually loses in weight, but microscopical investi- 

 gation shows that it still functions. A true atrophy of the 



