282 PHYSIOLOGY 



nervous system the main tracts cannot be shown to be functional 

 before the date at which they acquire their medullary sheaths, sug- 

 gesting that previously any impulse making its way along the tract 

 underwent dissipation before arriving at its destination. It is possible 

 too that the mvelin sheath mav serve as a source of nutrition to the 



*/ V 



enclosed axis cylinder, which, in the greater part of its course, is far 

 removed from its trophic centre, namely, the cell of which it is an out- 

 growth. This trophic function of the myelin sheath has a certain basis 

 of fact in that the myelin sheath is as a rule larger in those fibres which 

 take the longer course. 



