120 PHYSIOLOGICAL CHEMISTRY 



up and C0 2 is given off. Possibly the swelling and shortening 

 of the muscle fibers is caused by the acid and heat. The in- 

 dividual muscle fibers become shorter and thicker. The acid 

 is quickly destroyed, C0 2 is given off, and relaxation follows. 

 If the acid is not removed, cramps or rigor result. It has been 

 suggested that the swelling of the fibers is caused by taking up 

 water under the influence of acid, since proteins swell in acid 

 solution. The question is still unsettled, however. 



Brain and Nerves. Brain and nerve substance makes up a 

 far smaller proportion of body weight than do the muscles, but 

 the primary importance of this kind of tissue is obvious. The 

 brain and nerves are the directing mechanism which controls 

 the body activities. In higher animals, particularly in man 

 where intelligence is at the maximum, the brain is relatively 

 larger than in lower forms. The composition of brain and nerve 

 tissue is thus of great interest. This kind of tissue contains 

 less protein than the muscles. Gray matter contains 85+% 

 water, white matter 70+% water. The characteristic fact in 

 the composition of brain and nerve tissue is the presence of 

 large quantities of alcohol-ether soluble substances. These in- 

 clude cholesterol, phospholipins, cerebrosides, but almost no neu- 

 tral fat, possibly none at all. The principal phospholipins are 

 lecithin, kephalin and various myelins; of the cerebrosides the 

 most important are cerebron, phrenosin and kerasin. Various 

 extractives also are found, such as creatine, inosite, purine 

 bases, lactic acid and other substances. Caprine ( <x NH 2 

 caproic), an amino acid occurring in brain protein has been 

 found in no other tissue. Glycogen is not stored in the brain. 



The brain and nerves show a vigorous oxidative metabolism, 

 producing C0 2 . They are very sensitive to lack of oxygen, and 

 quickly cease functioning properly if their oxygen supply is 

 cut off. 



Nothing is known of the chemical changes involved in mental 

 activity. Since the lipins are present in such large amount, it 

 is possible that they are concerned in some way in these proc- 

 esses. 



