AND PHYSIOLOGY. 



173 



the muscles of the tongue and fauces it is forced into the 

 Pharynx. As soon as it fairly enters this passage, the mus- 

 cles by an involuntary movement seize it, and force it rap- 

 idly past the opening into the lungs, and at the same 

 moment the epiglottis is forced down upon the larynx, to 

 prevent its entrance into the trachea. This part of the pro- 

 cess is involuntary, from the necessity of keeping the passage 

 to the lungs open as much as possible, in order to admit air, 

 and also from the great danger of introducing any other sub- 

 stance. And so perfectly carried on is this function that it is 

 seldom — compared with the frequency of deglutition — that 

 even a fluid escapes the vigilance of this sentinel. 



312. Passage of the Food through the Esophagus. — The 

 food passes slowly through the Esophagus into the stomach, 

 it being forced along by the contraction of the muscular 

 fibers, aided by the oily secretion of the Esophageal glands. 



313. Gastric Digestion. — As soon as the food reaches the 

 stomach, the most important part of the process of digestion 

 commences, all the previous steps being preliminary. When- 

 ever any solid substance comes in contact with the inner or 

 mucous membrane of the stomach, it excites the gastric glands 

 to pour out in abundant quantity the Gastric fluid. 



314. Gastric Fluid— its Amount— Pepsin. — This fluid is 

 a transparent liquid of a little greater consistency than water, 

 and of a perceptibly acid taste. It possesses the property of 

 coagulating albumen, and of separating the whey or serum 

 from the milk in a very short time, and is secreted at the rate 

 of seventy ounces per day. This property, however, is owing 

 to a peculiar organic compound called Pepsin, which acts after 

 the manner of a ferment at the normal temperature of the 

 human body. The Gastric fluid also possesses antiseptic 



What part of the process is under, and what part is not under the control of the will? 

 812. How is the food carried through the Esophagus? 313. Where does the most im- 

 portant part of digestion take place ? What effect has any solid substance upon the Gas- 

 tric Glands of the Stomach ? 314. What are the properties of the Gastric Fluid? How 

 much is secreted daily ? What effect has it on the decay of substances ? 



