DIGESTION AND ABSORPTION 



303 



Peristaltic waves on the gullet of man. 

 (A bolus means little ball.) 



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

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

 These wavelike movements (called peristaltic movements) are 

 characteristic of other parts of the 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 " oc 

 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 add. 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- 



