588 VETERINARY PHYSIOLOGY 



is fundamentally an alteration in the course of metabolism is 

 indicated by Gaskell's observation that the vagus is an anabolic 

 nerve (p. 421). 



III. Chemical Regulation. 



Yet another factor plays an important part. The products 

 of the metabolism of one structure have an effect upon other 

 structures, and so co-relate and regulate their reciprocal 

 activity. 



It has already been shown that the carbon dioxide pro- 

 duced in muscle stimulates the respiratory centre (p. 527). The 

 slightest increase in the Ch of the blood increases the activity 

 of the kidneys (p. 580). A product of the activity of the 

 duodenum — secretin — has been shown to cause secretion by 

 the pancreas (p. 322). 



Chemical regulation plays a very important part in verte- 

 brate animals, and special organs have been evolved which 

 have as their function the elaboration of products which are 

 passed into the blood, each to produce definite and specific 

 actions in the body. 



Such structures may be called glands with internal secre- 

 tions, or endocrinetes. The name of hormones, from opixao) 

 " I excite," has been suggested for the internal secretions, but, 

 since they do not always increase functional activity, but may 

 check it, the name is unsuitable. 



Before deciding that any structure is a true endocrinete, 

 it is necessary to prove that it produces a specific product or 

 products with definite and specific actions. 



The evidence required is of various kinds. 



1. The effects of removal may be studied, and if these are 

 definite — 



2. The effects of transplanting a part of the structure in 

 preventing the onset of the changes may be tried. If this is 

 successful — 



3. The effects of removing the graft may be observed. 



4. The effects of administering extracts of the structure 



