182 Biological Chemistry. 



also been extensively employed in biological research for 

 the purpose of tracing out the various processes of synthesis 

 and degradation of fats in living objects. They warrant, 

 therefore, some more detailed description in this place. 



Extraction of Fats from Tissues. The fats are soluble 

 in organic solvents, and can therefore be separated from 

 the other constituents of tissues. In the case of the 

 animal tissues, the substances extracted by solvents such 

 as ether or light petroleum consist chiefly of fats ; the 

 latter are, however, almost invariably accompanied by 

 small amounts of other substances, some of which are 

 phosphorus derivatives, such as lecithin, and also a widely 

 distributed complex alcohol known as cholesterol. These 

 substances appear to have a great biological significance. 

 The chemical investigation is as yet far from complete, and 

 they are designated by the somewhat vague name of 

 " lipoids." In the case of plant tissues, ether and light 

 petroleum also extract fats and lipoids, but these are often 

 mixed with other substances such as essential oils, pig- 

 ments, glucosides, etc., of which the biological significance 

 is so little known that they may be excluded from dis- 

 cussion in this place. 



The ordinary animal tissue contains generally about 

 80 per cent, of water. Direct extraction of the moist 

 tissue by ether or light petroleum is not a satisfactory 

 process ; the tissue must be first dried and powdered before 

 the fat is extracted. Various methods are employed for 

 this purpose, the most satisfactory, perhaps, being the 

 heating of the tissue in vacuo at 100, as by this pro- 

 cedure the oxidation of the fat can be avoided. Owing to 

 the fact that "lipoids" are extracted at the same time, 

 the quantitative estimation of fat is not very simple.* If 



* Not only is the fat extracted by solvents mixed with lipoids, but the 

 quantitative estimation is complicated by the fact that certain of the substances 



