136 DIGESTION 



was produced even when the pancreatic nerves were cut. 

 Bayhss and Starhng showed that it occurred when not 

 only the pancreas but also the duodenum was separated 

 from the central nervous system. The mechanism, there- 

 fore, is entirely chemical. Neither acid alone nor extract 

 of duodenal mucosa alone on injection into the blood is 

 effective, but when the extract is first treated with HCl 

 and the mixture injected a profuse secretion from the 

 pancreas takes place. The substance thus formed, which 

 differs from a ferment in being thermostable, Bayhss and 

 Starhng called secretin, and the substance in the duodenal 

 mucosa from which it is produced they called prosecretin. 

 Secretin is the best-known example of a hormone or chemical 

 substance which, made in one organ, travels in the blood- 

 stream to stimulate another organ to activity. 



Since it is the acid of the gastric contents which causes 

 the formation of secretin, it would appear improbable that 

 the composition of the pancreatic juice changes in adapta- 

 tion to the diet, except in so far as the nature of the food 

 in the stomach alters the amount of acid secreted. 



BILE 



Bile, the secretion of the liver, is an alkahne, mucinous 

 fluid of which the principal constituents are bile-salts, 

 bile-pigments, cholesterol, lecithin and fats. It is con- 

 tinually being formed in the liver, from which it is secreted 

 into the intestine either directly or after a period of storage 

 in the gall-bladder. While in the gall-bladder it becomes 

 modified by the abstraction from it of water, and the 

 addition to it of mucin and nucleo-albumin. The signifi- 

 cance of the gall-bladder appears to be related to the fact 

 that bile is both a secretion and an excretion. As an excre- 

 tion it has to be removed from the liver as soon as formed, 

 owing to the toxic nature of the waste products which it 

 contains ; as a secretion it has to be passed into the intestine 

 at intervals owing to its digestive action. 



The bile-salts, which are sodium glycocholate and sodium 



