Chap. XV] DIGESTIVE PROCESSES 293 



any starch it may contain begins to be changed into sugar ; third, 

 it acquires a more or less alkaline reaction. 



Vomiting. — Under ordinary circumstances the contractions of 

 the cardiac sphincter muscle prevent the regurgitation of food, but 

 strong contractions of the stomach or spasmodic contractions of 

 the abdominal muscles may, if the diaphragm is fixed, force the 

 contents of the stomach through the oesophagus and mouth to the 

 exterior. This is called vomiting., 



CHANGES THE FOOD UNDERGOES IN THE STOMACH, OR 

 STOMACH DIGESTION 



Peristaltic action of the stomach. — The food which enters the 

 stomach is delayed there by the contraction of the sphincter muscles 

 at the cardiac and pyloric openings. The cavity of the stomach is 

 always the size of its contents, which means that when it is empty 

 it is contracted, but when food enters it expands just enough to 

 hold it. Within a few minutes after the entrance of food small 

 contractions start in the middle region of the stomach and run 

 toward the pylorus. These contractions are regular and become 

 more and more forcible as digestion progresses. As a result of 

 these movements the food is macerated, mixed with the acid gastric 

 juice, and reduced to a liquid mass called chyme. At intervals the 

 pyloric sphincter relaxes and the wave of contraction forces some 

 of the chyme into the duodenum. The fundal end of the stomach 

 does not take part in these movements, but serves as a reservoir 

 for food which is under slight pressure, as the muscles are in a state 

 of continual contraction or tone. Due to the lack of movement 

 and the muscular tone, the gastric juice cannot penetrate the bolus 

 of food, and the ptyalin with which it became mixed in the mouth 

 continues its action, and the digestion of starch continues for about 

 twenty minutes. As the chyme is gradually forced into the duode- 

 num, the pressure of the fundus forces the food into the pyloric 

 end. 



Time required for stomach digestion. — It is obvious that the 

 time required for gastric digestion depends upon the nature of the 

 food eaten. An average meal of mixed food requires about five 

 hours for gastric digestion. The ejection of chyme through the 

 pylorus occurs at regular intervals, and is supposed to depend 

 upon the consistency and acidity of the chyme. Solid particles 



