20 Feeds and Feeding. 



saliva, or 88 lbs., to which must be added 4.4 lbs. secreted during 

 rest, making 92.4 lbs. in all. The mingling of saliva with the food 

 aids the sense of taste by dissolving small quantities of food which 

 affect the nerve ends of the tongue. In this moist condition the 

 food is more easily swallowed. The most important property of 

 the saliva is due to the enzyme, called ptyalin, which it contains. 



Enzymes are mysterious organic compounds which are able to 

 change or break down other organic compounds without themselves 

 being broken down. 



30. Ptyalin. — The first enzyme of digestion, ptyalin, converts the 

 insoluble starches of food into a sugar called maltose. The pro- 

 teins and fats of food are not changed by the action of the saliva. 



Since most of the changes which food substances undergo during 

 digestion are effected thru enzymes, their general nature should be 

 early understood by the student, and ptyalin action serves as an 

 example. If a quantity of starch, placed in a dish, is treated with 

 saliva and the whole kept at body temperature, the starch so treated 

 will gradually dissolve, and after a time malt sugar will be found 

 in its stead. The complex starch molecule has been cleaved or split 

 into simpler ones by the action of the ptyalin. The enzyme caus- 

 ing this change is itself not altered in character or function, how- 

 ever, or seemingly exhausted in energy thereby, but is still capable 

 of changing more starch into sugar. So far as known, there is no 

 limit to the amount of sugar which a given quantity of ptyalin will 

 produce if the supply of starch is maintained and the resultant 

 sugar is continuously removed from the solution. If the saliva is 

 heated above 176° F., it will no longer possess this power. At the 

 temperature of ice water its action ceases, altho the enzyme is not 

 destroyed, for on warming it becomes active again. Acids destroy 

 ptyalin if added much beyond the point of neutrality. Each of the 

 several enzymes of digestion is capable of acting on only one of 

 the groups of nutritive substances— on either proteins, carbohy- 

 drates, or fats. Some act only in the presence of acids, and others 

 only in neutral or faintly alkaline solutions. All are most active at 

 about the temperature of the body. 



31. Digestion in the stomach. — The food remains but a compara- 

 tively short time in the mouth, and is then passed on thru the gullet 

 to the stomach, where it is acted on by the gastric juice. This con- 

 sists of water containing the enzymes, pepsin and rennin, and also 

 from 0.2 to 0.5 per ct. of hydrochloric acid, the gastric juice of car- 

 nivore, or flesh-eating animals, being more acid than that of others 



