DIGESTION AND ABSORPTION 



303 



by the presence of food in the tube, push the food down to the 

 stomach by a series of contractions until it reaches the stomach. 

 These wavelike movements (called peristaltic movements) are 

 characteristic of other parts of tho food tube, food being pushed 

 along in the stomach and 

 the small intestine by a 

 series of slow-moving mus 

 cular waves. Peristaltic 

 movement is caused by 



Peristaltic waves on the gullet of man. 

 (A bolus means little ball.) 



fcoZus of food. 



muscles which are not 



under voluntary nervous 



control, although anger, fear, or other unpleasant emotions have 



the effect of slowing them up or even stopping them entirely. 



Stomach of Man. The stomach is a pear-shaped organ 

 capable of holding about three pints. The end opposite to the 

 gullet, which empties into the small intestine, is provided with a 

 ring of muscle forming a valve called the pylorus. There is also 

 another ring of muscle guarding the entrance to the stomach. 



Gastric Glands. If we open the stomach of the frog, and 

 remove its contents by carefully washing, its wall is seen to be 

 thrown into folds internally. Between the folds in the stomach of 

 man, as well as in the frog, are located a number of tiny pits. 

 These form the mouths of the gastric glands, which pour into the 

 stomach a secretion known as the gastric juice. The gastric glands 

 are little tubes, the lining of which secretes the fluid. When we 

 think of or see appetizing food, this secretion is given out in con 

 siderable quantity. The stomach, like the mouth, " waters " 

 at the sight of food. Gastric juice is slightly acid in its chemical 

 reaction, containing about .2 per cent free hydrochloric acid. It also 

 contains two very important enzymes, one called pepsin, and an 

 other less important one called rennin. 



Action of Gastric Juice. If protein is treated with artificial 

 gastric juice at the temperature of the body, it will be found to 

 become swollen and then gradually to change to a substance 

 which is soluble in water. This is like the action of the gastric 

 juice upon proteins in the stomach. 



The other enzyme of gastric juice, called rennin, curdles or coag- 



