336 THE DUCTLESS GLANDS 



thymus atrophies move slowly in animals from whom the 

 testes have been removed. 



Henderson obtained similar results in rabbits, guinea -pigs, 

 and cattle. He states, further, that in bulls and unspayed 

 heifers the normal atrophy of the thymus which begins after 

 the period of puberty is greatly accelerated when the bulls 

 have been used for breeding, and when the heifers have been 

 pregnant for several months. The retarded atrophy is due, 

 according to Goodall, to a persistent growth of the lymphoid 

 tissue, a delay of the fatty invasion, and a delay in the process 

 of disintegration of the epithelium composing the Hassan's 

 corpuscles. 



According to Marrassini and Gellin, castration gives rise 

 to an enlargement of the thymus. Squadrini believes that 

 castration interferes with the normal involution of the gland. 

 From these observations it would appear that the normal 

 involution of the thymus is due to the development of the 

 reproductive organs, though this cannot be the only cause. 

 The experiments, further, tempt one to the hypothesis that 

 the thymus furnishes an internal secretion of some kind which 

 ministers to the needs of the economy before the reproductive 

 organs are fully developed. Normally this internal secretion 

 is provided by the testes (or ovary) after puberty, but if 

 castration is performed, the thymus maintains its original 

 structure and functions. This internal secretion must, of 

 course, be of a different nature from that which determines 

 the development of the secondary sexual characters, as these 

 do not become manifest in castrated animals. 



It must be added that recent careful experiments by Parke 

 cast considerable doubt upon the intimate relation between 

 testis and thymus. 



E. Physiological Effects of Extracts of Thymus 



Svehla found that the thymus of the ox during embryonic 

 life yields to extracts a substance which lowers the blood- 

 pressure. This he considers as a manifestation of an internal 

 secretion. From what has been said in an earlier chapter, 

 it is clear that this depressor substance is simply that unknown 

 material which is common to all animal tissues and organs. 

 It is not specific for the thymus. 



