PANCREATIC JUICE 



259 



the same significance in explanation of the phenomena 

 of secretion. In one i-espect there is a remarkable 

 difference between the pancreas and the salivary glands. 

 The latter always secrete as the result of a stimulus 

 reaching them along their nerves. It is not certain that 

 the cells of the pancreas are supplied with nerves at all 

 and it is certain that they can be made to secrete by a 

 chemical substance which is carried to them by the blood. 

 This hormone (p. 26) is called secretin and is manu- 



FiQ. 83.— A Portion of the Pancreas of a Rabbit. 



A, at rest ; B, in a state of activity. 



a, granular central zone of the cells ; 6, clear outer zone ; c, lumen of 

 alveolus ; d, junction of two neighbouring cells. 



factured in the cells of the mucous membrane of the 

 duodenum. The arrival of acid food from the stomach 

 into the duodenum causes the secretin to be discharged 

 into the blood by which it is carried to the pancreas. By 

 this mechanism a flow of pancreatic juice is insured for 

 the digestion of the food which arrives in tlie duodenum. 

 14. The Nature and Action of Pancreatic Juice.— 

 Pancreatic juice is a somewhat viscid fluid, alkaline from 

 the presence of sodium carbonate and containing a fairly 

 large amount of protein in solution. It contains further, 

 as its most important constituents, three soluble ferments. 



s 2 



