172 



HITCHCOCK'S ANATOMY 



of the waste Nitrogen of the system, and many salts, espe- 

 cially the Phosphates, which can be eliminated by no other 

 organ. The secretion of the kidneys is the urine, upon 

 the regular secretion of which the health of the system greatly 

 depends. 



FUNCTIONS OF THE DIGESTIVE ORGANS. 



310. Mastication; Use of the Tongue; Use of the Saliva. 

 Amount of Saliva, — The first process through which the food 

 must pass is Mastication, or reducing it to a pulp by means of 

 the teeth and admixture of the saliva. The service of the tongue 

 is to keep the food between the teeth and to place it in such a 

 position that it will readily receive the saliva. The saliva is 

 of use to moisten the food, since the gastric fluid will much 

 more readily dissolve it than if dry or solid. It aids articu- 

 lation and the sense of taste by keeping the lining of the 

 mouth in a moist and pliant state. It also is of use to cleanse 

 the mucous membrane, and by its moisture to quench or pre- 

 vent thirst. Air is also carried by it into the stomach to aid 

 the process of digestion. But the most important use of this 

 fluid is the conversion of starch into sugar. This property 

 depends mainly upon a peculiar organic active substance con- 

 tained in it called Ptyalin, and is most active when in a state 

 of incipient decomposition. The saliva secreted daily varies, 

 according to different authorities, from about three pounds to 

 six pounds and a half, and is alkaline in its character. Acid, 

 aromatic, and pungent substances increase the amount of the 

 secretion very much. 



311. Deglutition. A part of the Process involuntary. 

 — After mastication, the next process is that of swallow- 

 ing, or deglutition. The first step is to place the bolus, 

 or mouthful, upon the back part of the tongue, when by 



Of what great use are the Kidneys ? 310. Describe the process of Mastication. Of 

 what use is the Saliva ? How much is secreted every day ? What increases its amount? 

 811. Describe the process of swallowing. 



