LIPID CONTENT AND COMPOSITION OF ANIMAL 567 



of "obesity" tissue, as determined by densitometric analysis, as well as by 

 water analysis, is given in Figure 1 . 



Although analyses of the composition of the tissues of a number of animal 

 species have been reported, only those for normal adult rats will be given 

 here. These are recorded in Table 8. 



Although wide differences in fat content may occur in conjunction with 

 variations in nutritional condition, the values for normal rats (18 to 20%) 

 correspond roughly with the figures in man, namely 1-23%. 



(3) The Normal Composition of Animal Fats 



The fats which are laid down in the fat depots, as well as in supporting 

 tissues, are characteristic of the species of animal from which they are ob- 

 tained. Although the composition of these fat depots may be influenced to 

 some extent by extreme changes in the diet of the animal, animals of dif- 

 ferent species will, on the whole, synthesize entirely different types of body 

 fat when they are given the same basal diet. Moreover, the animal pos- 

 sesses an ability to produce, from the same basic foodstuffs, specific fats for 

 deposition in different organs. It has been known for a long time that the 

 fat laid down in the internal organs has a higher melting point and a lower 

 iodine value than has that deposited in the subcutaneous tissues of the 

 same animal. Thus, the animal must be able to manufacture simultane- 

 ously, from the same basic components, a number of types of fats in the 

 several organs. The nature of the body fat and the site at which it is de- 

 posited are likewise controlled by certain hormones; of these agents, the 

 sex hormones are probably the most important, since sex plays a most im- 

 portant role in the metabolism of fats. 



Many investigations on the subject of animal fats have been concerned 

 with determining the composition of the mixed fat from the animal as a 

 whole, rather than from a specific tissue. This is obviously the only prac- 

 tical solution for investigations of small organisms, when it is impossible to 

 effect an adequate separation of tissues and to obtain a sufficient supply of 

 material for an analysis. However, even this difficulty has been overcome 

 by the application of new technics for the mechanical separation of the 

 structural elements of the cell; these will be discussed in a later section (see 

 Section 6, page 603). 



A number of studies on the qualitative and quantitative aspects of fat 

 storage of animals as large as the rat have been made, employing a mixed 

 fat from the entire animal. These samples have been obtained either by a 

 rendering technic applied to the macerated tissues of the animal as a whole, 

 as employed by Anderson and Mendel, 49 or by the ether extraction of an 



