78 HUMAN PHYSIOLOGY 



one set of neurons, and at another with other adjacent 

 neurons. There is also some evidence that the dendrites 

 as a whole may expand and contract, and thus become 

 connected with those of adjacent neurons. 



Axon. The axis cylinder process, as it passes away from 

 the cell, becomes a Nerve Fibre, and acquires one or two 

 coverings. 



1. A thin transparent membrane is present in all peripheral 

 nerves, and has been called the neurilemma. Between it and 

 the axis cylinder a series of nuclei surrounded by a small 

 quantity of protoplasm, forming the so-called nerve cor- 

 puscles, is found at intervals. The mode of origin of these 

 is unknown. Fibres with only this sheath have a grey 

 colour, and may be called non-medullated fibres. They are 

 abundant in the visceral nerves. 



2. A thick white sheath the medullary sheath or white 

 sheath of Schwann which gives the white colour to most of 



Fio. 37. Pieces of two white Nerve Fibres. 



the nerves of the body, appears somewhat late in the develop- 

 ment of many nerve fibres. It lies between the primitive 

 sheath with the nerve corpuscles and the axis cylinder. It 

 is not continuous, but is interrupted at regular intervals at 

 the nodes of Ranvier (Fig. 37). It is composed of a sponge- 

 work or felt-work of a horn-like material neuro-keratin 

 the meshes of which are filled with a peculiar fatty material, 

 from which certain chemical substances have been isolated, 

 the relationships of which to one another are little under- 

 stood. The most abundant of these has been called pro- 

 tagon. It yields stearic acid ; hence it is allied to the fats, 

 and it contains nitrogen and phosphorus. Its constitution 

 is not known. Along with protagon, or as a result of its de- 

 composition, lecithin occurs. This is a fat in which one of 

 the acid radicles is replaced by phosphoric acid linked to 

 cholin Hydroxyethyl-trirnethyl-ainmonium-hydroxide. 



