THYMUS. 143 



Much of the above description is based on the work of Mall. 

 For further details the reader is referred to his articles on the 

 subject. 



4. THYMUS. 



The thymus is a gland-like organ found only in embryos 

 arid young animals. It undergoes with age a retrogression, and 

 at the twentieth year in man we find only a connective-tissue 

 vestige of it. 



In the first years of life the thymus consists of various 

 lobes, which are made up of lobules 0.5 to 1 cm. in diam- 

 eter, joined together by connective tissue. These lobules con- 

 sist of smaller lobules about 1 mm. in diameter, which are 

 separated by connective-tissue septa. The smaller lobules 

 are made up of adenoid tissue, which is richer in blood-vessels 

 and lymphocytes at the periphery than at the centre. We 

 may thus distinguish a dark cortical substance and a light 

 medulla. 



The thymus is of epithelial origin, and develops later its 

 adenoid structure. It is an outgrowth from the foregut of the 

 embryo, and, according to the majority of authorities, the 



FIG. 101. 



Connective 



Section through a secondary lobule of the thymus of a child six months old. < 50. 



Hassal's concentric corpuscles which are found in the medullary 

 substance are the remains of this epithelial tissue (Stieda, His, 

 Maurer, etc.). These corpuscles (Figs. 101 and 102) consist at 



