Digestion in the Stomach. 169 



Rennet and Rennin. Rennet, used in cheese making, is a familiar 

 substance obtained from the fourth stomach of the calf. When milk 

 enters the stomach it is curdled ; that is, the casein previously dissolved 

 in the liquid milk is curdled. This curdling, or coagulation, is caused 

 by a substance in the gastric juice called rennin. 



Churning Action of the Stomach. At the same time all 

 the food is soaked by the gastric juice, the process being 

 greatly assisted by the churning motion of the stomach 

 caused by the action of the muscular coat. This muscular 

 action of the stomach is called the peristaltic action. The 

 food is thus gradually reduced to a pulpy mass called 

 chyme. During the first part of digestion in the stomach 

 the thick ring of circular fibers, called the pylorus (gate- 

 keeper), around the opening from the stomach into the 

 intestine, keeps the passage nearly closed, leaving a small 

 hole for liquids only. But s~the food is reduced to the 

 proper condition the muscles relax and allow the chyme to 

 pass into the intestine. And at last any indigestible sub- 

 stances are usually allowed to pass. 



Sphincter Muscles. Such rings of muscular fibers as 

 the pylorus, guarding openings, are called sphincter 

 muscles. 



Time of Stomach Digestion. The time required for the 

 stomach digestion of a meal is from three to four hours, 

 though this may be much longer if very indigestible sub- 

 stances have been eaten, or if the condition of the body 

 or mind is such as to retard the process of digestion. 



Chyme. The rest of the food, now called chyme, is 

 passed on into the small intestine. It is acid, and in a 

 liquid or .semiliquid condition. Chyme, as it enters the 

 intestine, is a mixture of digested, partly digested, and 

 undigested materials. Some of the starch has been changed 



