CHAPTER XIX. 



DIGESTION. 



Digestion converts food into forms which can be dissolved in 

 water, or absorbed and utilized by the body. The digestive 

 organs vary in size, shape, and capacity with different kinds of 

 animals. Some animals, such as dogs, fowl, pigs, and men, have 

 short digestive organs, adapted only to concentrated foods, such 

 as meat, cereal grains, etc. The digestive organs of other 

 animals, such as sheep, goats, cows, etc., are large and adapted to 

 bulky food containing small amounts of nourishment. Horses 

 and hogs have smaller digestive organs. 



Outline of Digestive Process. The first step in digestion is 

 preparation of the food by chewing or grinding. This usually 

 takes place in the mouth. The food is there moistened with 

 saliva, a slightly alkaline liquid, which not only softens the food 

 and lightens the labor of chewing, but contains an enzyme termed 

 ptyalin which converts starch into sugar. Most of the work of 

 digestion is performed by enzymes, substances which have the 

 power of transforming other substances into simpler substances 

 without themselves being changed. Ptyalin acts only in an 

 alkaline medium, and as soon as the food becomes acid by fermen- 

 tation or by means of the gastric juice, its action stops. Grinding 

 or other preparation of the food before feeding will partly de- 

 crease the labor of chewing. Grinding is especially necessary 

 for pigs, or for other animals when small hard seed are fed. 

 When whole grain is fed to cattle, it is sometimes imperfectly 

 masticated, and a considerable number of grains passes through 

 undigested. 



The sheep and the ox have four stomachs. The first and sec- 

 ond stomachs are used to store the food until it is returned to 

 the mouth for a second mastication. The food then passes to the 

 third stomach, which has a sieve-like structure, where the food is 

 kneaded and ground up. The digestion takes place in the fourth 

 stomach. 



These animals are called ruminants, and are able to utilize 



