650 PHYSIOLOGICAL CHEMISTRY. 



(ash) by weighing the residue. Some of the elements which are left 

 in the inorganic residue have, however, been actually constituents of 

 organic compounds ; iron, for instance, w r hich is left in the ash, has 

 been chiefly a constituent of haemoglobin ; sulphur, left as a sulphate, 

 may have been a constituent of albumin, etc. 



The complex nature of the various organic matters has been referred 

 to in the preceding chapter, and will be more fully considered below; 

 but it may be mentioned here, that some of these organic substances 

 (or groups of substances) may be separated by a successive treatment 

 of the animal matter with various solvents. Thus, by treating with 

 ether or carbon disulphide, all fats may be extracted ; by then treat- 

 ing with alcohol and water successively other substances (generally 

 termed extractive matter or extractives) are dissolved, which may be 

 obtained by evaporating the solution. 



The relative quantities of the three constituents in some of the 

 animal fluids and tissues is shown in the following table : 



Among the extractives are found creatine and creatinine, urea, uric 

 acid, organic salts, etc. After the fatty matter and the extractives 

 have been removed there remains an elastic and somewhat horny 

 mass, which consists chiefly of proteins (albumin, fibrin, globulin, etc.). 



The complete separation of all substances is extremely difficult on 

 account of the great similarity in properties of many of these sub- 

 stances, and the rapid changes which they suffer when acted upon by 

 solvents or chemical agents. 



As the nature or composition of many of the inorganic salts present 

 in the animal tissues is changed during the burning off of the organic 



1 The metals in combination with the biliary acids are not included. 



