LIPID CONTENT AND COMPOSITION OF ANIMAL 569 



in which widely varying reactions occur other than oxidation, the lipids are 

 much more prominent, and occur in amounts varying from one-sixth to a 

 quantity equivalent to that of protein. 



The nature of the lipids which are present as components of the various 

 organs varies markedly from that of the depot fats. The lipids in the or- 

 gans contain practically no neutral fat, but only phospholipids, cerebrosides, 

 and cholesterol. On the other hand, the depot fats are almost entirely com- 

 posed of neutral fats. The depot fats are present in different amounts, ac- 

 cording to the diet and nutritional condition. 



Phospholipids have consistently been shown to comprise the largest pro- 

 portion of the essential lipids. According to Kaucher and her collabora- 

 tors, 252 cerebrosides are second in importance in skeletal and cardiac mus- 

 cle, followed by cholesterol, which is a minor constituent. In smooth 

 muscle such as that of the stomach and intestine, the cholesterol fraction is 

 more important than are the cerebrosides. The above investigators like- 

 wise reported that sphingomyelin was of greater importance in the soft 

 tissues of the body than it was in the skeletal musculature. 



The composition of the lipids present in the various organs of rats fifteen 

 and seventy days of age is summarized in Table 9, while a further analysis of 

 the phospholipid components is included in Table 10 (pages 570, 571). 



The proportion of the several tissues composed of essential lipids in- 

 creases with age. The highest concentration of lipid occurs in the brain, 

 where the essential lipids increase from 29.82% at fifteen days to 42.68% at 

 seventy days. The testes show the next greatest percentage change; in 

 these organs the lipid increases from 13.73% at fifteen days to 20.41% at 

 seventy days. The phospholipids make up the major portion of the essen- 

 tial lipids. The increase in proportion of this component is consistent, in 

 all tissues, with increasing age. This accounts for most of the rise in total 

 essential lipids. The cerebrosides are of paramount importance, especially 

 in the brain, where they increase from 3.77 to 8.42% of the total lipids dur- 

 ing the fifty-five day increment in age between the two series of tests. 



There are considerable variations, from a qualitative standpoint, in the 

 composition of the phospholipids in the several tissues. In most instances, 

 the cephalins increase markedly between the ages of fifteen days and sev- 

 enty days, so that they equal or exceed the proportion of choline-contain- 

 ing phospholipids at the later age period. In a majority of cases, the cho- 

 line phospholipids decline with increasing age, a change which is largely to 

 be ascribed to a decrease in the lecithin fraction. 



