NERVE 57 



a horn-like substance — neuro-keratin — the meshes of which 

 are filled with a peculiar fatty material. 



The nerve fibres run together in bundles to constitute 

 the nerves of the body, and each bundle is surrounded by 

 a dense fibrous sheath, the perineurium. When a bundle 

 divides, each branch has a sheath of perineurium, and in 

 many nerves this sheath is continued, as the sheath of Henle, 

 on to the single fibres which ultimately branch off" from 

 the nerve. 



Not only do nerves branch and anastomose in the great 

 nerve plexuses, but in the nerves themselves a similar plexus- 

 like rearrangement of the bundles and fibres takes place. 



Fig. 20. — Pieces of two white Nerve Fibres. 



Each nerve fibre ends in a series of dendritic expansions, 

 which vary greatly in character according to the structures 

 to which they pass. 



III. Chemistry of Nerve. 



The chemistry of neuron cells and tlieir processes has been 

 deduced from a study of the chemistry of the grey matter of 

 the brain where they preponderate, while the chemistry of the 

 white fibres is indicated by the analysis of the white sub- 

 stance of the brain, which consists chiefly of meduUated fibres. 



The grey matter contains over 80 per cent, of water. 

 The solids consist of rather less than 1 per cent, of proteins. 

 Two globulins, one coagulating at a low and the other at a 

 higher temperature, and a nucleo -protein have been isolated. 

 Lecitliin and cholesterol each constitute about 3 per cent. 



The white matter contains only about 70 per. cent of 

 water. The proteins, similar to those in the grey matter, 

 constitute between 7 and 8 per cent. Lecithin occurs in 



