126 THE INDIVIDUAL ORGANISM 



dilute hydrochloric acid in a mortar to break up the cells, neutralized and 

 filtered the product, and injected some of the resulting fluid into the 

 veins of another experimental animal, a copious flow of pancreatic juice 

 ensued. The substance responsible was named secretin. 



In the period since secretin was thus discovered a great amount of 

 information has been gained about hormones, and a field of science called 

 endocrinology has come into being. Various glandular structures that had 

 long been known to be in some way essential to life, though not part of 

 any recognized physiological system, now were recognized as endocrine 

 glands producing important hormones. Other organs such as the pancreas, 

 the functions of which had seemed simple and well understood, were 

 found to have additional endocrine functions. 



There still remain a number of glandular tissues of uncertain function ; 

 investigation of these and the search for additional hormones that may 

 be produced by better known glands is still in progress. In spite of all 

 that has been learned, we are still very far from a complete understanding 

 of hormones and their mode of action. 



The deciphering of the body's endocrine control is beset with special 

 difficulties. For one thing, the amount of hormone secreted by a gland 

 (although sufficient to produce stimulation or inhibition of bodily func- 

 tion) is very small, and it is difficult to obtain a sufficient quantity for 

 study. Moreover, many of the endocrine glands are located deep within 

 other vital tissues and are difficult to reach surgically without killing the 

 experimental animal. Much of our present information has had to be 

 obtained by removing some gland suspected of producing a hormone 

 from an experimental animal and then comparing the functioning of this 

 animal with one that remains intact. If the animal that has had a gland 

 removed shows specific symptoms, and if these symptoms can be allevi- 

 ated by feeding or injecting the animal with extracts taken from the same 

 glands of other individuals, the existence of an endocrine function is 

 indicated. The next step is to obtain a sufficient quantity of the secretion 

 and to isolate the essential substance. If possible, this substance is puri- 

 fied, and its chemical composition and structure are determined. In the 

 case of adrenalin, thyroxin, testosterone, and other important hormones, 

 such knowledge is now sufficient to permit synthetic manufacture. These 

 and other hormones that must at present be obtained from the glands of 

 fish or sheep or other animals are already an important part of the 

 "medicines" available to the modern physician. 



Another difficulty in determining the role of any given hormone is 

 that the endocrine glands appear to be particularly susceptible to the 

 influence of other hormones. Removal or disease of one endocrine gland 

 may markedly affect the functioning of another that has no evident 

 connection with it. Investigation of the pituitary gland, for example, is 



