74 NERVOUS TISSUE. 



mixed nerves, grow toward the developing anterior roots, and 

 eventually mingle with them to form the mixed nerves. As 

 development proceeds, the bipolar ganglion-cells become gradu- 

 ally transformed in most vertebrates by the shifting of the two 

 neurons, into unipolar cells ; but in many fibers the cells remain 

 permanently bipolar. 



" The ganglia on the sympathetic and on other peripheral 

 nerves are formed from small masses of neuroblast-cells, which 

 separate off from the rudiments of the spinal ganglia and give 

 origin to neurons and dendrons much in the same way as do the 

 neuroblasts within the central nervous system. 



" The manner in which the medullary sheath and neurolemma 

 of the nerve-fibers are formed is not well understood. The neuroglia- 

 cells appear to be developed from cells which are at first similar 

 to the neuroblasts, but, in place of giving off a neuron and den- 

 drons, a number of fine processes grow out from the cell in all 

 directions, forming the fibers of the neuroglia." 



Chemistry of Nervous Tissue. The following is the 

 analysis of the brain of an ox by Petrowsky : 



Gray Matter. White Matter. 



Water . 81.60 per cent. 68.30 per cent. 



Solids 18.40 " " 31.70 " " 



100.00 100.00 



The percentage composition of the solids is as follows : 



Gray Matter. White Matter. 



Proteids 55.37 24.72 



Lecithin 17.24 9.90 



Cholesterin and fat 18.68 51.91 



Cerebrins 0.53 9.55 



Other organic compounds (including neurokera- 



tin and protagon) 6.71 3.34 



Salts 1.45 0.57 



Halliburton divides the solid constituents of the nervous tissues 

 into the following classes : 



a. Proteids. These comprise a very considerable percentage 

 of the solids, especially in the gray matter (over 50 per cent.). 



b. Neurokeratin and nuclein. 



c. Phosphorized constituents, especially protagon and lecithin. 



d. Cerebrins. Nitrogenous substances' of unknown constitu- 

 tion. 



e. Cholesterin. Especially abundant in white matter. 



/. Extractives. Creatin, xanthin, hypoxanthin, inosit, lactic 

 acid, leucin, uric acid, and urea. 



g. Gelatin and Fat. From the adherent connective tissue. 



h. Inorganic Salts. The total mineral matter varies, according 

 to different writers, from 0.1 to 1 per cent. 



