22 FEEDS AND FEEDING, ABRIDGED 



or less souplike. On being reswallowed, this finely divided material 

 usually flows along the gullet past the slit, and directly into the third 

 stomach, from which it passes into the fourth, or true stomach. Water 

 or liquid food, when first swallowed, may not be forced thru the slit 

 into the paunch but may pass at once to the third stomach. 



Enzymes. — As most of the changes which food undergoes in diges- 

 tion are effected thru enzymes, their general nature should be clearly 

 understood. Enzymes are mysterious organic compounds which are 

 able to change or break down other organic compounds without them- 

 selves being changed or broken down. To illustrate the action of 

 enzymes, we will take ptyalin, the enzyme contained in the saliva, 

 that converts the starch of the food, which is insoluble, into sugar, 

 which is soluble. If starch is mixed with saliva and the whole kept at 

 body -temperature, the starch gradually dissolves, being changed to 

 sugar. Thru the action of the ptyalin, the complex starch molecule has 

 been cleaved, or split, into the simpler molecules of sugar. If starch 

 is mixed merely with water, instead of saliva, this change will not 

 occur. 



The ptyalin is not itself altered by this process, for, if more starch is 

 added and the resulting sugar removed, the process may be repeated 

 many times. However, heating the enzyme above a certain tempera- 

 ture destroys it. At freezing temperature its action ceases, tho the 

 enzyme is not destroyed, for on warming it becomes active again. 

 Ptyalin acts best in a neutral or slightly alkaline solution and is 

 destroyed by the presence of much acid, while some other enzymes act 

 only in acid solutions. Each of the enzymes of digestion is capable of 

 acting on only one of the groups of nutrients — on proteins, on carbo- 

 hydrates, or on fats. 



Digestion in the mouth.— Not only is the food prepared for swallow- 

 ing in the mouth, but in most animals the .first step in digestion occurs 

 here, thru the action of ptyalin on the starch in the food. The saliva 

 of cattle and dogs, however, contains little or no ptyalin, and that of 

 horses but little. The saliva of swine contains a fair amount, and that 

 of man, monkeys, rabbits, rats, and mice has the greatest starch digest- 

 ing power. 



Digestion in the simple stomach.— With such animals as the horse 

 and pig, which have simple stomachs, the food passes directly from the 

 mouth thru the gullet to the single stomach. The glands in the walls 

 of the stomach secrete the digestive fluid called gastric juice. This 

 contains the enzymes pepsin and rennin, and from 0.2 to 0.5 per ct. of 

 hydrochloric acid. If the food became acid as soon as it reached the 

 stomach the action of the ptyalin of the saliva would cease at once. 

 However, the first part of the stomach does not secrete any acid, 



