266 



THE INTERNAL ENVIRONMENT OF THE BODY 



Part III 



Fig. 15.10. Effects of parathyroid deficiency. A, The normal dog before the 

 operation. B, The same animal the second day after the removal of the para- 

 thyroids in a convulsive condition with twitching muscles. C, The same animal 15 

 minutes after calcium was given to it. The parathyroid glands regulate the amount 

 of calcium and phosphorus in the body. (Courtesy, Turner: General Endocri- 

 noloiyy. Philadelphia, W. B. Saunders Co., 1948.) 



The secretion of the medulla is controlled by the autonomic or involuntary 

 nervous system. The sympathetic fibers of the autonomic nervous system 

 produce a hormone, sympathin, which apparently is distributed to tissues 

 whenever impulses pass over the nerve fibers to them. The reactions to 

 sympathin are similar to those of adrenalin but the two substances are evi- 

 dently entirely separate. The colors of many fishes are due to pigment cells 

 or chromatophores. Their expansion and contraction are controlled partly 

 by nervous impulses and partly by hormones such as sympathin and adrenalin. 



Cortex. The adrenal cortex produces hormones, certain of which are 

 essential to life; if the cortex of both adrenal glands is entirely removed, an 

 animal dies within a few days. Experiments suggest that there are three groups 

 of these hormones, all of which are steroids and some of which have been 

 synthesized. The first group (called the desoxycorticosteroids) controls the 



Posterior 

 vena cova 



Aorta 



Blood vessel 



Capsule 



Fig. 15.11. Human adrenal glands. Left, A gland caps each kidney but is not 

 a part of it. Right, A section of an adrenal. The medulla is within the cortex like 

 the filling in a sandwich. The two parts are different in origin and function; the 

 cortex is essential to life, the medulla is not. 



