MECHANISM OF GASTRIC SECRETION 31 



dog hardly any parotid saliva is formed, but the submaxillary, 

 supplying mucin which lubricates the food for swallowing, 

 is very active. When meat is given in the form of a dry powder 

 the parotid is as active as the submaxillary, giving a secretion 

 which is not Only abundant but contains mucin. In contrast 

 to this, strong stimulation of the mucous membrane by dis- 

 agreeable, non-edible substances gives rise to an activity of 

 all the glands, but the saliva is poor in mucin, since the sub- 

 stance is not to be swallowed. 



In psychical secretion the same adaptation holds good, for 

 it is only necessary to pretend to throw the pebbles or sand 

 into a dog's mouth in order to bring out the same differences 

 in the saliva. 



The condition of the salivary centre has an important 

 influence on psychical secretion. A hungry animal responds 

 easily, while in an animal just fed no response is obtainable. 

 Fear or embarrassment or other emotions also affect the salivary 

 centre, inhibiting the normal reflexes. In man the large salivary 

 glands are active intermittently, but the smaller glands are 

 always active. 



Gastric Glands. Two kinds of cells are present in the 

 gastric glands, known as chief cells and border cells. The former 

 are alone present in the pyloric end of the stomach, and it 

 has been stated by Heidenhain that this portion of the stomach 

 produces no acid. The pyloric end, carefully resected and con- 

 verted into a blind pouch, is alkaline in reaction. This, by exclu- 

 sion, leaves the border cells as the source of the hydrochloric 

 acid of the gastric juice. During the activity of the gastric 

 glands histological changes take place, especially in the chief 

 cells, similar to those already described in the salivary glands. 

 They 'also have a double supply of cranial and sympathetic 

 nerves. Stimulation of the vagi after a latent period of from 

 four and one-half to ten minutes gives a distinct flow. The 

 delay is due to the simultaneous irritation of inhibitory fibers. 

 Stimulation of the sympathetic gives no result. 



Mechanism of Gastric Secretion. In the investigations 

 leading to an understanding of the manner in which the gastric 

 juice is normally secreted, operative procedures have been 

 most fertile in results. A fistulous opening through the abdomi- 

 nal wall leading into the stomach cavity, or, better still, into 



