INTERNAL SECRETIONS OR HORMONES 409 



ovaries has been described as relieving certain of the nervous 

 symptoms which supervene on their removal or atrophy. It 

 has further been found that ovarian substance, when given to 

 dogs whether male or female, causes an increase in the rate of 

 protein metabolism, although no similar action is found with 

 testicular substance. There is thus evidence that the ovaries, 

 like the thyroid, form an internal secretion having an import- 

 ant action in accelerating the metabolism. It is at least 

 probable that the testes produce a similar substance. 



6. Thymus. This structure develops as an epithelial out- 

 growth from one or more of the branchial arches of the embryo. 





FIG. 165. Section of the lobules of the Thymus to show the lobules, 

 with Hassall's corpuscles in the central part. 



Round these outgrowths masses of lymph-like tissue collect, and 

 thus a much lobulated structure lying in the front of the neck 

 and upper part of the thorax is formed. As development 

 goes on the epithelial core of each lobule breaks up and forms 

 nests of cells which are often disposed concentrically and form 

 the corpuscles of Hassall. These lie in the loose lymph-like 

 tissue, the medulla of the lobule, the meshes of which are 

 formed from the epithelial cells, and this is surrounded by a 

 cortex of more dense lymph-like 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 



