LIPIDS PRESENT IN MUSCLES 737 



were 1.53, 1.35, 1.26, and 1.34, respectively, as contrasted with figures of 

 2.05 and 1.82 in the two samples of smooth muscle. 



A relatively high proportion of liquid (unsaturated) fatty acids to solid 

 (saturated) fatty acids is present in muscle phospholipid. The proportion 

 found by Snider 201 was 73 for the liquid acids to 27 for the saturated acids. 

 This is much higher than the 55:40 proportion reported for liver. 202 The 

 constancies of these ratios have been confirmed by Sinclair. 203 The iodine 

 absorption value for the liquid fatty acids of muscle averaged 173, which 

 is lower than that found for the corresponding fraction in the liver (2.22). 



In the various muscles examined, the cerebrosides are the highest in 

 salmon and codfish, where they amount to 3.96 and 2.64%, respectively. 

 Cerebrosides are the lowest in the smooth muscles of the stomach and in- 

 testine. Neutral fat reaches the highest level in pork and the lowest in 

 the frog; however, the essential lipids are highest in the latter animal. 



(3) The Effect of Age on Muscle Lipids 



Whereas practically every tissue of seventy-day rats examined by Wil- 

 liams and his co-workers 74 had a higher lipid content than did those of 

 fifteen-day rats, skeletal muscle was unique in presenting the opposite 

 picture. The total lipids of the skeletal muscle of fifteen-day rats averaged 

 27.79%, while those of the rats sacrificed fifty-five days later amounted to 

 only 15.84%. However, during this period, the essential lipids had in- 

 creased from 8.21 to 12.40%; this is a rise of 51%. The decrease in total 

 lipids resulted from a decline in neutral fat from 19.58% at fifteen days to 

 3.44% at seventy days. The muscle was found to differ from all other 

 tissues examined in that the fat content in the young animal was high. 

 Another especially noticeable change was the increase in cerebrosides, 

 which amounted to 146% (1.45 to 3.57%). In the seventy-day rat, only 

 the brain and testes had a higher cerebroside content than did the skeletal 

 muscle. These changes occurred in spite of the fact that the total propor- 

 tion of the body weight composed of muscle tissues increased from 24% at 

 birth to 41% at the age of seventy days. 204 In contradistinction to the 

 conditions existing in skeletal muscle, the fat content of cardiac muscle in- 

 creases somewhat as the rat matures, and falls in line with the changes in 

 most other tissues during the growth interval from fifteen to seventy days. 



201 R. H. Snider, J. Biol. Chem., 116, 503-510 (1936). 



2 ° 2 R. H. Snider and W. R. Bloor, J. Biol. Chem., 99, 555-573 (1933). 



203 R. G. Sinclair, J. Biol. Chem., Ill, 261-273 (1935). 



204 H. H. Donaldson, The Rat, Memoirs Wistar Inst. Anat. Biol., 2nd ed., Philadelphia, 

 1924. p. 184. 



