522 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



some extent, to the sterols in the spinal cord. In still other situations, 

 the lipids may represent foodstuffs undergoing oxidation to serve as a 

 source of energy for the animal as a whole. The liver is the best example of 

 this type of organ. 



The tissue lipids exercise a number of varied functions. One usually 

 considers that the primary function of tissue fat, particularly in the so- 

 called fat depots, is to serve as a reserve source of calories. Another most 

 important role played by fats is that of an insulator, to prevent too rapid 

 loss of heat from the surface of the body or too great an absorption of heat 

 from the surrounding environment. Fatty tissues likewise serve as cush- 

 ions for the bony projections, and in this way pad the bones; they act to 

 protect the nerves and organs from shock. In addition, they serve as 

 anchoring tissue to fill in and occupy any space otherwise vacant. They 

 protect and hold the blood vessels in position. Finally, they have a most 

 important function in providing the necessary essential material for the 

 production of the semipermeable cell membranes of the blood cells, as well 

 as of the cells of most fixed tissues and organs. A very complete and 

 satisfactory treatise on the tissue lipids is included in the monograph of 

 Bloor. 1 



2. Theoretical Considerations in the Deposition of Lipids in 

 Different Species 



In 1936, Hilditch and Lovern 2 proposed a theory to explain the diver- 

 gence in the depot fats laid down by various species of animals. This 

 theory was based upon a simplification in the structure of fats which con- 

 formed to the increasing complexity of the animal. Thus, the greatest 

 variety of fatty acids are found in fats from the simplest marine forms; 

 these contain not only the saturated Cu, Ci6, and Cis acids, but also un- 

 saturated acids varying in chain length from Ci 4 to C22 and even, in some 

 cases, to C24. On the other hand, the higher land mammals contain as 

 the major fatty acids in their depot fat only three representatives, namely 

 palmitic, stearic, and oleic acids. In the case of animals whose evolu- 

 tionary development is intermediate between the most primitive and the 

 most complex form, the fatty acid makeup represents a distribution some- 

 what simpler than that of the marine animals, but with more acids than 

 occur in the higher land mammals. Hilditch and Lovern 2 were of the 

 opinion that these differences were not adventitious, but were the result of 



1 W. R. Bloor, Biochemistry of the Fatty Acids, Reinhold, New York, 1943. 



2 T. P. Hilditch and J. A. Lovern, Nature, 137, 478-481 (1936). 



