454 GENERAL CONSIDERATIONS 



between individuals and not between species. Such individual differ- 

 ences may be the material basis of evolution. 



499. Rhythmicity. — All animal activities seem to be rhythmic in 

 character. The nature of the organization seems to result in a gradual 

 acceleration of metabolism to a maximum and then a recession, followed 

 by another acceleration and recession. Regular periods of rest and sleep 

 are very general phenomena among animals. The rhythms which 

 accompany the function of reproduction are particularly evident, but 

 other functions of the body exhibit rhythms which are less pronounced. 



500. Uses of Foods. — As has been previously stated (Sec. 44), 

 food is necessary to replace waste, provide material for growth, and 

 furnish energy to the organism. It follows then that the food must be 

 made up of the same substances of which the body is composed or else 

 of substances readily transformable into them. The following sum- 

 marizes the present views on the uses of food in the body of a higher 

 vertebrate. 



1. The protein of food replaces the protein of the body which is used 

 up. 



2. All of the protein is used as it is secured, none of it being stored. 

 An excess is burned and the products of combustion eliminated. In 

 this process a certain amount of heat is developed, but proteins are not an 

 economical source of heat. 



3. Carbohydrates are mainly used in the body as sources of muscular 

 energy, being incorporated in the organization of muscles and then 

 broken down during muscular activity. 



4. The oxidation of carbohydrates also produces heat and more 

 than does the oxidation of protein but not so much as is obtainable by the 

 oxidation of fats. 



5. Fats are the most economical sources of heat in the body. 



6. The body maintains at all times in the liver a large store of carbo- 

 hydrates, from which a constant supply is furnished to the blood for use 

 by the muscles. 



7. An excess of both carbohydrates and fats is changed to fat and 

 stored as such. The fat taken into the body is not always stored in the 

 same form as the fat which is taken in but is broken down in the process 

 of digestion and recombined to form the various fats characteristic of the 

 particular organism. 



8. Salts are necessary elements in food because they serve to facilitate 

 chemical changes, create conditions which determine the solubility of 

 various substances, and, through the part they play in osmosis, partici- 

 pate in the transfer of water from one part of the body to another. 



9. Water is important as a solvent, as a carrier of substances in solu- 

 tion, and as a regulator and distributor of heat. The last function is 

 possible because of the high specific heat which water possesses. 



