328 FOOD AND DIGESTION 



ing tissue, the greater portion of the food being utilized in 

 the production of energy. 



236. Food Constituents. — Food in general is composed of 

 different kinds of material, every one of which is separately 

 digested and absorbed in the animal and which serves 

 special functions in the body. These constituents are: 

 Carbohydrates, fats, proteins, and inorganic salts, not to 

 mention water which, strictly speaking, should be considered 

 a food constituent; but since it is combined with all food 

 constituents and since its presence is necessary for their 

 solution and absorption, it will not be necessary to consider 

 it separately. 



237. Digestion. — The processes by which insoluble food 

 materials are rendered soluble for absorption into the blood 

 of animals are called digestion. Digestion takes place in the 

 various parts of the alimentary canal, which consists prin- 

 cipally of the mouth, stomach, and small intestine. There 

 are connected with these various parts of the alimentary 

 canal certain appendages, which are necessary for the various 

 activities of the food canal. Since digestion occurs in three 

 places and under three different kinds of conditions, it is ad- 

 visable to separate the discussion of these processes into three 

 parts: Salivary, gastric, and intestinal digestion. 



(a) Salivary Digestion. — The first process is one of masti- 

 cation, which serves to grind the solid food into more or less 

 fine particles so that the digestive juices can act on theni to 

 better advantage. While the food is being masticated, there 

 is poured into the mouth from three different glands a liquid 

 called saliva, which serves mechanicallv to combine the 

 fine particles of food together so that they may be more 

 easilv swallowed, and also to act chemicallv on some of the 

 food constituents. Saliva is a slightly turbid, opalescent, 

 somewhat viscid liquid which is composed almost entirely 

 of water, but with some soluble organic matter, inorganic 

 salts, and an enzyme called 'ptyalin. It is slightly alkaline 

 in character, due to the presence of sodium carbonate. The 

 enzyme ptyalin or salivary amylase is the active digestive 

 agent in the mouth. It acts on starch, changing it first to 

 dextrin and then to maltose. The process is hydrolytic in 



