INTERNAL SECRETIONS 505 



mentally demonstrated. The active principle of the suprarenal gland 

 adrenalin produces certain well-known effects ; for instance, it pro- 

 duces a rise of blood-pressure. 



From Fig. 238 it will be seen that, after the nerves to the thyroid 

 gland have been stimulated, the injection of the same amount of 

 adrenalin in the same time gives a greater rise of blood-pressure than 

 it did before the stimulation of these nerves. This is the more re- 

 markable, since a second injection of adrenalin, without stimulation 

 of the thyroid nerves, normally gives a smaller rise of blood-pressure 

 than before. This increased action of adrenalin is abolished if the 

 thyroid be excised before its nerves are stimulated, but is obtained 

 Avhen extracts of the thyroid gland are injected into the animal just 

 previously to the dose of adrenalin. 



These experiments, then, give definite evidence of the interworking 

 of the secretions named, and it is probable that the other secretions 

 also interact, some in an anabolic and some in a katabolic direction. 



The Organs of Generation Testes. The testis is generally regarded 

 as a "double " gland, consisting (1) of germinal cells, which form 

 the external secretion or elements concerned in reproduction; (2) of 

 interstitial cells, which form an internal secretion intimately connected 

 with the general metabolism of the body and the acquisition of second- 

 ary male characteristics. The view that the interstitial cells act as 

 a separate gland is not accepted by all authorities, but the following 

 facts go to support the view. It is stated that, when the vasa deferent : a 

 are tied, the germinal cells atrophy, but the interstitial cells persist. 

 Exposure to X rays is known to render animals impotent, but does 

 not change their secondary sexual characteristics. Histological 

 examination in such cases shows that the generative cells are de- 

 stroyed, but the interstitial cells are not damaged. 



The effects of castration have long been known. As a direct 

 local result the animal is rendered impotent, and the accessory sexual 

 apparatus atrophies. Generally, the sexual development of the 

 animal is arrested; the so-called secondary sexual characteristics are 

 not acquired (Figs. 239, 240); the general metabolism is so affected 

 that the animals tend to laziness and fatness. An excessive growth of 

 bone is also frequently brought about. These results are due to the 

 cutting off from the body of the internal secretions of the glands. It 

 makes no difference whether the seminal vesicles be left or removed. 



The results of transplantation experiments show that the secondary 

 male characteristics may be developed by this means e.g., the spurs 

 and comb of the cock, or the large thumb of the male frog. If the 

 testicle be transplanted in infancy, no spermatozoa develop, but the 

 gland becomes composed of large quantities of interstitial cells, and 

 the secondary male characteristics are acquired. It is stated that 

 a testicle so transplanted into a female tends to give her body male 

 characteristics. 



The internal secretion of the testes has been held to exert marked 

 influence upon the general health and mental and muscular activity 

 of the individual. It is claimed that a substance spermin (C 5 H U N 2 > 



