METABOLISM 61 



animals the fats and carbohydrates yield a ready 

 source of energy in the form of heat. Whether they 

 are always directly oxidized in the animal body with- 

 out ever having become part of the tissue itself is 

 perhaps questionable, but this seems to be true of 

 the carbohydrates if not of the fats. The liver 

 functions in an important way in the carbohydrate 

 metabolism of Vertebrates. The digested sugar is 

 transformed into another carbohydrate called gly- 

 cogen (" animal starch "), and stored up in the liver, 

 and later in the muscles, in the form of granules. 

 The glycogen is dissolved and given back to the blood 

 stream as the body requires it between meals, or is 

 oxidized in place, to release the energy involved in 

 muscular work. 



Similarly, the fats are stored up in the different 

 parts of the body or in special organs (the fat-body 

 of the frog, e.g.) to be drawn on as need arises. If 

 neither fats nor carbohydrates are available, the 

 proteins in the blood stream or even those of the 

 tissue itself may be broken down to supply the 

 necessary energy. Hence, the fats and carbohydrates 

 are often spoken of as the " protein-sparing " sub- 

 stances. 



In the digestive tract of the higher animals the fats 

 are split into their components, glycerine and fatty 

 acid, through the action of the enzyme, lipase. 1 Being 

 absorbed in this form they are recombined in the epi- 

 thelial cells or within the capillaries and circulate as 



1 See page 83. 



