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 myelin sheath may serve as a source of nutrition to the 

 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. 



