312 DIGESTION 



Secretion of saliva under natural conditions 



The secretion of saliva appears to be mainly controlled by the nervous 

 system. There are two general modes of nervous stimulation of salivary 

 flow. (1) The presence of materials in the mouth leads to the secretion 

 of saliva. There is a remarkably purposeful variation in the composition 

 of saliva depending on the nature and mechanical state of the material 

 present in the mouth. Thus a watery secretion is produced in the pres- 

 ence of dry powder, whereas acid leads to the secretion of a fluid high 

 in mucin, which would tend to neutralize the acid. (2) Stimulation of 

 other organs of sense, aside from that of taste, also leads to salivation. 

 We are all familiar with the experience of our mouths watering when we 

 smell or see a particularly tasty food. It is obvious that one must have 

 had the experience of tasting the particular food at one time and that 

 the stimulation due to smell or sight must have been acquired then. 

 Such an acquired stimulation is known as a conditioned reflex. 



"When the body is exposed to a situation of water loss, the salivary 

 secretion is depressed. As a result the mouth becomes dry, and the person 

 experiences the sensation of thirst. A normal human adult secretes 

 1-1.5 1. of saliva per day. 



After the food has been prepared in the mouth for further digestion, it is 

 swallowed and passes through the esophagus to the stomach. 



GASTRIC DIGESTION 



In the stomach the foods are mixed with the gastric juice. The gastric 

 movement renders the food creamy and semifluid in consistency. This 

 mass is then known as the chyme. It passes out of the stomach through 

 the pyloric opening into the duodenum. 



Gastric juice is secreted by three main types of cells. The secretion 

 of mucous cells is high in mucin, the parietal cells contribute hydrochloric 

 acid, and the zymogenic cells supply the zymogen, pepsinogen. In con- 

 trast to other fluids of the body, gastric juice has a very acid reaction, 

 e.g., pH 1.5-1.9, in the case of man. It has been estimated that the 

 secretion of parietal cells is 0.16A^ hydrochloric acid. If only hydrochloric 

 acid were secreted in the stomach," the pH would be around 1 ; however, 

 the other secretions contain substances such as proteins and sodium bi- 

 carbonate which partially neutralize the hydrochloric acid. The unneu- 

 tralized portion of the acid can be determined by titration with alkali 

 and is known as the free acid of the gastric juice (0.05-0. lA^ HCl) , while 

 the sum of the neutralized and the free acid is called the total acid. 

 When food is mixed with the gastric juice still more of the free acid is 

 neutralized, and the acidity of the chyme is pH 3-5, depending on the 

 nature of the food. 



