THE ANIMAL BODY DIGESTION METABOLISM 21 



the gullet past the slit, and directly into the third stomach, from which 

 it passes into the fourth or true stomach or it may re-enter the paunch 

 and then pass on into the other stomachs. Water or liquid food may pass 

 at once to the third stomach. 



Animals do not ruminate during sleep, while working, when excited, 

 or if in pain. The fact that the ox requires 7 to 8 hours daily for rumina- 

 tion handicaps him somewhat as a work animal, for if his hours of work 

 are long he must dispense with sleep to masticate his food. 



37. 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 themselves being 

 changed or broken down. To illustrate the action of enzymes, we will 

 take, as an example, 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 malt 

 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 temperature 

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

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

 lin 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; for example, on proteins, on 

 carbohydrates, or on fats. 



38. Digestion in the mouth. Not only is the food prepared for swal- 

 lowing 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. Recent 

 investigations have shown that the saliva of cattle and dogs contains 

 practically 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 digesting power. 



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

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

 mouth, where it remains but a comparatively short time, thru the gullet 

 to the single stomach. There it is acted on by the gastric juice, which 

 consists of water containing the enzymes, pepsin and rennin, and from 

 0.2 to 0.5 per ct. of hydrochloric acid. 



Pepsin, which acts only in weak acid solutions, converts the very com- 

 plex proteins into soluble and simpler, tho still complex, products known 



