CHAPTER XXVIII 

 INTERNAL SECRETIONS 



When we think of the means by which coordination is 

 secured in the body the picture of the nervous system at 

 once rises before us. It is true that the quick adaptive 

 reactions by which emergencies are met proceed from the 

 flight of nerve-impulses. But it is a fact much better 

 appreciated now than a few years ago that chemical 

 compounds borne from place to place by the blood have a 

 great deal to do with internal adjustments. The subject 

 may be called that of the internal secretions or, using a 

 term we have already employed, the hormones. 



A hormone is a substance produced in a definite locality 

 but having its effect elsewhere, perhaps at a great distance. 

 At least two examples which have been mentioned should 

 be recalled. One is the secretin formed in the lining 

 of the duodenum and taking effect upon the pancreas and 

 other digestive glands. A different hormone, gastric 

 secretin, is believed to arise in the lining of the stomach 

 under the influence of secretagogues and to be the chief 

 cause of the late formation of gastric juice in the normal 

 period of digestion. 



Another internal secretion of which it was necessary 

 to speak in an earlier chapter is that contributed by the 

 pancreas to the circulation and essential to the oxidation 

 of sugar. The want of this hormone is responsible for 

 diabetes. We must now add to these examples several 

 others. It may be said at the outset that a greater 

 degree of confusion exists in this department of physiology 

 than in any other. This is partly owing to the newness 

 of the recognition of hormones but is partly inherent in the 

 subject itself. Fresh discoveries are being reported and 



377 



