CHEMICAL CHANGES IN PLANTS AND ANIMALS. 645 



in the construction of new tissue. When the quantity of food absorbed is 

 greater than is required for repair and energy, the carbohydrates are converted 

 into fat, and this, with the excess of fat from the food, is stored up in the 

 fatty tissue of the body, to be drawn upon whenever needed. In starvation no 

 tissue decreases as much as the fatty. The fatty tissue of the animal body is a 

 depot where, during proper alimentation, nutritive material of great value is 

 stored, to be given off as it may be needed. 



Nutrition. In the process of nutrition five phases may be distin- 

 guished, viz. : Digestion, absorption, anabolism, catabolism, and excre- 

 tion of waste products. These processes are commonly spoken of 

 collectively as metabolism. 



Digestion is the process of converting food material into dialyzable 

 compounds, or into other forms of matter capable of absorption. Ab- 

 sorption is the mechanical process of transferring the digested mate- 

 rials from the alimentary canal into the circulation. Anabolism 

 includes the synthetic changes taking place after they are absorbed 

 until they have become a part of living cells. Catabolism includes 

 those destructive changes which take place chiefly in consequence of 

 oxidation, the oxygen being supplied during the process of respira- 

 tion. Excretion of waste products is the discharge of that material 

 which is no longer needed in the system. 



Digestion. It has been stated before that foods are divided into 

 two classes, inorganic and organic, and that the latter are subdivided 

 into proteins, carbohydrates, and fats. As a rule, the inorganic foods 

 are taken into the body without chemical change. Before the organic 

 foods can be absorbed they have to undergo digestion. This is the 

 process by which organic compounds capable of acting as foods are so 

 altered that they may be absorbed. The process of digestion will be 

 fully considered in a later chapter. 



Absorption, anabolism, catabolism, excretion. While these subjects, 

 particularly absorption and excretion, are considered later under the 

 various organs concerned (see Index), a brief statement of the changes 

 in the various food-stuffs, subsequent to their absorption, is made 

 here. 



Carbohydrates. The carbohydrates, mainly as dextrose, are carried 

 from the intestine by the portal system to the liver, where the bulk 

 of them is dehydrated and converted into glycogen. Some of the 

 dextrose is changed into glycogen by the muscle, and in this organ 

 and the liver a reserve-supply of glycogen is stored up. This gly- 

 cogen forms the most readily available source of energy for the body. 

 When used it is first split into dextrose, and then oxidized to carbon 

 dioxide and water, passing probably through a lactic acid stage. The 



