§ 5.52 CONTROL OF SECRETION OF METABOLIC HORMONES 253 



needed for cold-blooded vertebrates and even for birds, the study 

 of w^hich is still far behind that of mammals. 



Although the present survey has been kept intentionally brief, 

 it still makes clear that the situation is far from simple, even within 

 single classes of vertebrates. It is no longer possible to generalize 

 about even so closely related an order of mammals as the carni- 

 vores; for the ferret differs sharply from dogs and cats, and yet 

 resembles rats and man, in secreting its diabetogenic hormone from 

 the adrenal cortex instead of from the pancreas (§ 5.211). 



The mode of action of metabolic hormones involves problems 

 of biochemistry outside the scope of the present volume ; but the 

 discovery that the stereochemistry of sugar molecules can affect 

 their responsiveness to insulin is a notable advance in this field 

 (§5.212). 



The long-term nature of their actions distinguishes most 

 metabolic hormones from the quick-acting kinetic hormones, and 

 relates them in nature as v^ell as in function to the morphogenetic 

 hormones (Part II). In fact, there is an obvious overlap between 

 them, since for instance the supply of sugar to the tissues and the 

 stimulation of protein synthesis are essential precursors of growth, 

 and are often controlled by the same hormones. 



5.52 CONTROL OF THE SECRETION OF METABOLIC 

 HORMONES 



The three ways in which hormone secretion can be controlled 

 are all to be found acting upon different examples of metabolic 

 hormones (Table 31). 



5.521 Direct control of endocrine glands 



Direct control by physical or chemical means, other than nerve 

 impulses or hormones, has been seen already in the case of the 

 kinetic hormones of the vertebrate gut. One of the clearest examples 

 was secretin, the production of which is stimulated by the presence 

 of acid in the gut ; it then induces a flow of bicarbonate into the 

 gut until the acid is neutralized (§ 4.111). Similarly the secretion of 

 both the antidiabetogenic hormone, insulin, and the diabetogenic 

 hormone, glucagon, is brought about by the sugars in the blood. 

 A high concentration of glucose stimulates the insulin secretion, 



