ANIMAL LIFE AND ANIMAL PRODUCTS. FOODS 



429 



Animal Nutrition. Since there is continual loss, there must 

 be continual replacement. The animal resembles the plant, in 

 the fact that it can take up into its system only dissolved material. 

 It differs from the plant, however, in the fact that it is provided 

 with a wonderful laboratory in which insoluble substances are 

 changed into soluble ones. This is the digestive tract, consisting of 

 the mouth, stomach, and intestine. The production of soluble 

 substances of suitable composition is called digestion. 



The processes are too complex for detailed treatment here. Only 

 a few typical illustrations can be given. The principles concerned 

 have all been used and illustrated already, and many of the facts 

 are contained in previous chapters. 



Foods.-}- First, let us examine the table showing the percentage 

 composition of the edible portion of several articles of food: 



* The emulsified fat separates slowly as the cream; the protein (casein, colloidally suspended 

 in the skim milk) is coagulated by rennet and constitutes cheese; the carbohydrates (lactose, a 

 sugar) are then left in the water, along with inorganic salts. 



We note, at once, that there is little more water in milk than in 

 cod; that the animal foods, except milk (carrying lactose, p. 401), 

 contain no carbohydrates; that potatoes and corn when dried 



