INTERNAL SECRETIONS 



389 



an important action in accelerating the metabolism, and 

 it is at least probable that the testes produce a similar 

 substance. 



6. Thymus. This structure develops as an epithelial 

 outgrowth from one or more of the branchial arches of the 

 embryo. Round these outgrowths masses of lymph' tissue 



FIG. 153. Section of the lobules of the Thymus to show the lobules, 

 with Hassall's corpuscles in the central part. 



collect, and thus a much lobulated structure lying in the front 

 of the neck and upper part of the thorax is formed. As de- 

 velopment goes on the epithelial core of each lobule breaks 

 up and forms nests of cells which are often disposed concen- 

 trically and form the corpuscles of Hassall. These lie in a 

 loose lymphoid tissue, the medulla of the lobule, and this 

 is surrounded by a cortex of dense lymphoid tissue. The 

 thymus is largest in relationship to the body weight about 

 the time of birth, but it continues to grow, although not in 

 proportion to the growth of the body, till about the age of 

 puberty. After about twenty-four years of age it atrophies 

 and is replaced by a mass of fatty tissue. The Hassall's cor- 

 puscles seein to atrophy earlier than the lymphoid tissue. 

 Castration in cattle and guinea-pigs markedly retards the 



