CHAPTER XVI. 



NERVOUS TISSUE. 



IN common with the other solid tissues of the body, nervous 

 tissue contains a large amount of water. The percentage of water 

 present depends upon the particular form of nervous tissue but in 

 all forms it is invariably greater in the gray matter than in the 

 white. Embryonic nervous tissues also contain a larger percentage 

 of water than the tissues of adult life. The gray matter of the 

 brain of the foetus, for instance, contains about 92 per cent of water, 

 whereas the gray matter of the brain of the adult contains but 

 83-84 per cent of the fluid. 



Among the solid constituents of nervous tissue are proteins, chol- 

 esterol, cerebrin, lecithin, keplwlin, protagon (f), paranucleopro- 

 tagon, nuclein, neurokeratin, collagen, extractives and inorganic 

 salts. The proteins are present in the greatest amount and com- 

 prise about 50 per cent of the total solids. Three distinct proteins, 

 two globulins and a nucleoprotein, have been isolated from nervous 

 tissue. The globulins coagulate at 47 C. and 70-75 C. respec- 

 tively, while the nucleoprotein coagulates at 56-60 C. This nuc- 

 leoprotein contains about 0.5 per cent of phosphorus (Hallibur- 

 ton, Levene). Nervous tissue is composed of a relatively large 

 quantity of a variety of compounds which collectively may be 

 grouped under the term " lipoid " substances resembling the fats 

 in some of their physical properties and reactions but distinct in 

 their composition. We will class cerebrin, cholesterol and the 

 phosphorized fats, as " lipoids." 



The group of phosphorized fats are very important constituents 

 of nervous tissue. The best known members of this group are 

 lecithin, protagon (?) and kephalin. Lecithin occurs in larger 

 amount than the other members of the group, has been more thor- 

 oughly studied than the others and is apparently of greater impor- 

 tance. Upon decomposition lecithin yields fatty acid, glycero-phos- 

 phoric acid and choline. Each lecithin molecule contains two fatty 

 acid radicals which may be those of the same or different fatty 

 acids. Thus we have different lecithins depending upon the particu- 



248 



