CHAPTER XXI 

 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, choles- 

 terol, cerebrosides (cerebrin, etc.), lecithin, kephalin, protagon (?), para- 

 nucleoprotagon, nuclein, neurokeratin, collagen, extractives, and inorganic 

 salts. The proteins are present in the greatest amount and comprise 

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

 globulins, and a nucleoprotein, have been isolated from the nervous 

 tissue. The globulins coagulate at 47C. and 70-7 5C., respectively, 

 while the nucleoprotein coagulates at 56-6oC. This nucleoprotein 

 contains about 0.5 per cent of phosphorus (Halliburton, 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 cholesterol, the cerebrosides and the phosphorized fats as lipoids. 



The consideration of lipoids (or lipins 1 } is assuming added impor- 

 tance. These substances constitute one of the two great groups of 

 tissue colloids, the proteins being the remaining group. So far as struc- 

 ture and chemical properties are concerned the various classes of lipoids 

 are entirely unlike. 



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 thoroughly studied 

 than the .others and is apparently of greater importance. Upon de- 

 composition lecithin yields fatty acids, glycero-phosphoric acid, and 



1 Rosenbloom and Gies: Biochemical Bulletin, i, 51, 1911. The term lipoid was intro- 

 duced by Overton (Studien iiber die Narkose, Jena, 1901, Gustav Fischer). 



370 



