SALIVARY SECRETION. 39 



further or pyloric end of the stomach and after being thor- 

 oughly broken down by this movement and partially digested 

 by the pepsin of gastric juice, is passed on in portions into 

 the duodenum, where it meets with the secretions of the pancreas 

 and liver. These secretions, acting along with auxiliary juices 

 secreted by the intestine itself, ultimately bring most of it into a 

 state suitable for absorption. What the digestive juices leave 

 unacted on bacteria attack, especially in the caecum, so that by 

 the time the food has gained the large intestine it has been di- 

 gested as far as it can be. In its further slow movement along 

 the large intestine the process of absorption of water proceeds 

 rapidly. 



Disturbances in the digestive process may be due not only to 

 possible inadequacy in the secretion of one or other of the diges- 

 tive juices, but also to disturbances in the movements of the 

 digestive canal. Such disturbances will not only prevent the 

 forward movement of the food at the proper time, but, by failing 

 to agitate the food, they will prevent its proper admixture with 

 the digestive juices, for of course an enzyme acts more rapidly 

 when the mixture is kept thoroughly agitated with the food than 

 when it is stagnant. 



Digestion in the Mouth. 



Salivary Secretion. In the mouth, besides its preparation 

 for swallowing, by mastication, etc., the food, mainly on account 

 of its taste and smell, stimulates sensory nerve endings which, 

 by acting on nerve centres, set agoing several of the digestive 

 secretions. The first of these is the secretion of the salivary 

 glands. On account of their ready accessibility to experimental 

 investigation, very extended studies have been made of the sali- 

 vary glands, and from these studies some of the most important 

 physiological truths, concerning the nature of the nervous con- 

 trol of glands in general, have been drawn. Of the three salivary 

 glands in man, the parotid secretes a watery saliva usually con- 

 taining the enzyme, ptyalin, and the submaxillary and subling- 

 ual secrete a sticky saliva containing mucin, usually along with 

 some ptyalin. When the glands are not secreting, the cells that 



