1 6 NUTRITION OF FARM ANIMALS 



3. FATS AND RELATED BODIES THE LIPOIDS 



28. Classification. Under the rather vague term " lipoids," 

 or fat-like substances, there are included, besides true fats, a 

 large number of chemical individuals of varied and complex 

 molecular structure. Chemically, these substances (with the 

 exception of the cholesterins) are characterized by containing 

 radicles of the so-called fatty acids, principally the higher ones 

 of the series. Physically, the lipoids have been defined, prin- 

 cipally from the standpoint of the physiological chemist, as 

 substances which are soluble in organic solvents, such as ether, 

 alcohol, chloroform or benzol. This latter definition, however, 

 includes substances, such as the cholesterins, which would be 

 excluded by the chemical definition just given. For the present 

 purpose, the principal lipoids may be conveniently grouped under 

 five heads : (i) fats, (2) waxes, (3) cholesterins, (4) phosphatids 

 or phospholipins, (5) cerebrosids or galactolipins. 



The Fats 



29. Occurrence. It is a familiar fact that the bodies of 

 animals contain a not inconsiderable amount of fat, the per- 

 centage seldom falling below six in the very lean animal while it 

 may rise as high as forty in the very fat animal. The fat is 

 the reserve material of the body and is contained in what is 

 called adipose tissue (94) , consisting of cells of connective tissue 

 more or less filled with fat. Larger or smaller amounts of adi- 

 pose tissue are found in all parts of the body but especially in 

 the subcutaneous tissues, the tissues surrounding the intestines, 

 and, particularly in fat animals, in the muscles. 



In plants, fats are usually less abundant. They occur in 

 all parts of the plant but are especially stored up in the seeds, 

 where they serve as reserve material which is metabolized 

 during germination. Some seeds, like those of cotton, flax 

 and rape, contain fat so abundantly that they are commercial 

 sources of oil. In the plant, the fat is not deposited in special 

 tissues but is usually distributed through the protoplasm of 

 the cell. Both animal and vegetable fats are mixtures of 

 various simple fats, often containing also small amounts of 

 free fatty acids. 



