THE DIGESTIVE SYSTEM. 175 



During the first part of digestion in the stomach the thick 

 ring of circular fibers called the pylorus (gatekeeper) around 

 the opening of the stomach into the intestine keeps the 

 passage nearly closed, leaving a small orifice for liquids 

 only. But as the food is reduced to the proper condition 

 the pyloric muscles relax and allow the chyme to pass into 

 the intestine. And at last any indigestible substances are 

 usually allowed to pass. 



Sphincter Muscles. Such rings of muscular fibers, 

 guarding openings, are called sphincter muscles. There 

 is a similar one at the anal opening. 



Time of Stomach Digestion. The time required for 

 the digestion of any ordinary meal is from three to four 

 hours, though this may be much longer if very indigestible 

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

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



Absorption from the Stomach. Some parts of the 

 food that are already digested, or such matters as are sol- 

 uble, e.g. water containing sugar, peptone, salts, etc., may 

 be absorbed immediately through the walls of the mouth 

 and stomach into the blood capillaries. Recent experiments 

 show that the amount of absorption from the stomach is 

 much less than was formerly supposed ; water, for instance, 

 " when taken alone, is practically not absorbed at all in the 

 stomach. As soon as water is introduced into the stomach 

 it begins .to pass out into the intestine, being forced out in 

 a series of spurts by the contractions of the stomach." 



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 un- 



