THE DIGESTIVE SYSTEM. 169 



medium, not at all in an acid medium, or in too strong an 

 alkaline medium ; 4, that it has almost indefinite power, 

 if the product of its own action (sugar) is not suffered to 

 accumulate. In all these respects, with the exception 

 of the third, the salivary ferment resembles ferments in 

 general, which are destroyed by heat, delayed by cold, 

 and are limited in their action only by the accumulated 

 product of such action." WALLER. 



Enzymes. Ptyalin is a type of a group of bodies 

 called unorganized ferments, or enzymes. These ferments 

 are the agents that produce the peculiar chemical changes 

 that are the chief part of digestion. 



Mucous Glands and Mucus. Besides the salivary 

 glands, there are great numbers of simple glands in the 

 mucous membrane lining the mouth. These secrete a 

 glairy substance called mucus. 



Experiments with Digestive Liquids. It may be proved by 

 experiment that saliva turns starch to grape sugar in an alkaline solu- 

 tion and at the proper temperature. Also that pepsin dissolves proteids 

 in an acid (hydrochloric) at the right temperature. The proteid is 

 turned to peptone, and becomes soluble and diffusible, capable of 

 absorption through the walls of the stomach and intestine. We find 

 that the different elements of the pancreatic juice can, in alkaline solu- 

 tion, and at the right temperature, emulsify fats, turn proteid to pep- 

 tone, and convert starch into grape sugar. 



The Pharynx. The cavity back of the mouth, beyond 

 the soft palate, is the pharynx. The pharynx is a funnel- 

 shaped cavity, communicating above with the passages 

 from the nostrils ; in front it opens into the mouth ; below 

 it connects with the windpipe, through the glottis, and 

 with the gullet, which, as we have seen, lies just back of 

 the windpipe. 



