Chemical Changes in Animal Organism. 277 



which can be extracted in a Soxhlet apparatus (see p. 183) 

 by ether or light petroleum. The so-called lipoids are 

 extracted at the same time. The substance is generally 

 dried before extraction. The amount of protein can be 

 ascertained by the determination of nitrogen by Kjeldahl's 

 method (see p. 35). The number thus obtained multi- 

 plied by the factor 6*45 gives approximately the amount 

 of protein. The carbohydrates are generally estimated by 

 difference. The above process gives a rough approxima- 

 tion of the amounts of the three chief constituents in a 

 given foodstuff, and is generally sufficiently accurate for 

 most metabolic experiments. The majority of the standard 

 foods have been repeatedly submitted to analysis, but in 

 many cases different samples show a varying composition 

 thus, the amount of fat in meat is a very variable quantity. 

 The following are the constituents of some of the chief 

 articles of diet : 



Milk, which is the standard food of the young, con- 

 tains protein, fat, and carbohydrate. The chief protein is 

 caseinogen, which is readily precipitated from the diluted 

 skimmed milk on acidification with acetic acid. A certain 

 amount of fat not separated in the skimming process is 

 carried down with the precipitate, and can be extracted by 

 means of graded strengths of alcohol and then with ether. 

 The greater part of the fat from milk can be separated 

 by centrifugalization, and forms the chief constituent of 

 butter. After separation of the fat and caseinogen a 

 residue is obtained which contains the milk-sugar, which 

 can be separated in a crystalline form after concentration, 

 and also a small amount of a water soluble protein (lactal- 

 bumin). On treatment of milk with extract of the mucous 

 membrane of the calf's stomach, which contains a ferment 

 known as rennin (or rennet), a clot is produced (junket) 

 as the result of a change the nature of which is not accu- 



