r-|i\r. MI nir.KSTION 139 



(hid" sugar; arc canr.-stigar, grape-sugar, malt-sugar, 

 ami milk-sugar. 



Cellulose, found in fruits, cereals, and all vegetables, forming 

 the walls of the plant cells. 



3. Fats. Found in milk, butter, cheese, meat, various 

 oils. 



4. Salts. The salts present in foods arc very much the 

 -.aim- as those found in the body. The chief are the chlorides, 

 phosphates, and carbonates of sodium and potassium, and, to a 

 smaller extent, of calcium and magnesium, with salts of iron 

 and of certain organic acids. 



5. "Water. Present in all foods. 



The body of an animal consists of substances of the same 

 kind as those present in its food, that is, it consists of pro- 

 tcids, carbohydrates, fats, salts, and water. An animal must 

 be supplied with certain elements, namely, those found in the 

 body. The only element which it can take up in the free state 

 is oxygen, and that by the lungs only ; the other elements 

 must be received united with one another in the form of definite 

 chemical substances, the food-stuffs. Proteids supply nitrogen, 

 carbon, and hydrogen, carbohydrates and fats supply carbon 

 and hydrogen bat no nitrogen. (They all three contain oxygen, 

 but this, being already combined in them with other elements, 

 is not, like the oxygen taken in by the lungs, available for 

 oxidation.) Hence an animal can live on proteids, salts, and 

 water alone (with oxygen derived from the air), for by these 

 it obtains all the elements it requires, and in the proteids the 

 elements arc united in a form which it can use for its nourish- 

 ment. lUit it cannot live on carbohydrates and fats, for these 

 contain no nitrogen ; moreover, the carbohydrates and fats arc 

 not essential, since the carbon and hydrogen which they supply 

 may also be obtained from proteids. An animal can live 

 without carbohydrates and fats, but it must have proteids. 

 Nitrogen is only taken up by the body in the proteids taken 

 as food, and proteids may therefore be called the nitro- 

 genous food-stuffs. Some carbohydrates and fats can be 

 made artificially out of simpler substances but with difficulty 

 and at great expense ; proteids have never been made arti- 

 ficially at all. Plants have the |x>wcr of making all these 

 nut of simpler substances, out of salts, carbonic 



