Human Physiology, 



serves which preside over digestion \ to bring every part be- 

 tween the mashing and grinding surfaces of the teeth ; to feel 

 if it is reduced to a soft pulp ; and then to convey the food to 

 the back part of the mouth, where the action of involuntary 

 muscles performs the act of swallowing. The food passes quickly 

 down the channel of the esophagus into the stomach passing 

 over the opening of the windpipe, which is carefully closed by 

 a valve constructed for that purpose opening for every breath, 

 closing tightly at every swallow. 



When this pulp of food arrives in the stomach, which is a 

 bag or pouch, small when empty, but expanding with its con- 

 tents, it is turned over and over by the contractions of muscles 

 along and around it. At the call of the nerves the blood- 

 vessels swell and fill, and thousands of glands separate from the 

 blood, or rather manufacture out of it, the gastric juice which 

 dissolves food, and reduces it to a creamy whitish liquid called 

 chyme. 



Fig. 39 represents two glands in the coat of the stomach 

 highly magnified. They are tubes ex- 

 panding into cells, resembling vegetation. 

 In all glands the forms give a great 

 expanse of membranous surface, and in 

 that surface, or in the nerves expanded 

 on the surface, lies the power of taking 

 the required elements from the blood, 

 and making from them the required sub- 

 stance. Human gastric juice consists of Fig " 39GLANDs IN THE 



.. , - COAT OF THE STOMACH 

 994.40 thousandths Of water, a little of a Magnified 45 diameters. 



nitrogenous element called pepsine, some 



common salt, and a very small trace of hydrochloric acid. Of 



this gastric juice there is every day formed large quantities, as 



much, some observers have estimated, as twenty or thirty pounds, 



which, having performed its use, is again absorbed into the 



circulation. 



