392 THE MAINTENANCE OF THE INDIVIDUAL 



metabolism ; (3) growth ; (4) reproduction ; and (5) the master center 

 which serves as the "generahssimo" of all the endocrines. Such an 

 arrangement means that a gland which produces more than one 

 hormone may have to be considered in more than one category. 



Regulators of Digestive Processes 



Little can be added to the description of the hormone secretin 

 except to point out that physiologists are somewhat uncertain as to 

 whether the hormone is first produced as an inactive substance called 

 prosecretin, or as the active agent known as secretin. The action of 

 this internal co-ordinating mechanism may be seen, for example, 

 in the secretion of the pancreatic juice which is always poured at 

 apparently just the proper time into the small intestine. At first, 

 this co-ordination was believed to be due to some sort of undiscovered 

 nervous reflex mechanism that was stimulated as the food passed a 

 given point. This interpretation was discounted by two promi- 

 nent English physiologists, Bayliss and Starling, who in 1902 showed 

 that it is the passage of the acidulated food past the pylorus into the 

 upper part of the small intestine {duodenum) which stimulates the 

 production of a hormone, secretin. This substance is absorbed by 

 the blood and carried throughout the body, the portion reaching the 

 pancreas furnishing the necessary stimulus to effect the release of its 

 digestive enzymes. 



Regulators of General Metabolism 



Adrenals 



The paired adrenal gland is composed of an outer cortex and an 

 inner medulla, each part having a different embryonic origin and pro- 

 ducing a different hormone. In the lower vertebrates the cortex is 

 represented by an elongated mass of glandular tissue called the 

 interrenal, lying between the kidneys and derived embryologically 

 from the lining of the body cavity. The medulla on the other hand 

 is at first a separate structure, composed of so-called chromaffin 

 cells, which have their origin in the nervous tissue of the autonomic 

 nervous system. In the higher vertebrates the interrenal and the 

 chromaffin cells become incorporated to form the adrenal gland. 



The outer portion, or the cortex of the adrenals, secretes a hormone 

 known as cortin which has been proved to be essential to life. If 



