252 PHYSIOLOGY 



The non-medullated fibres (Fig. 102) differ from the medullated simply in the 

 absence of a medullary sheath. They possess, in many cases at any rate, a primitive 

 sheath, under which we find nuclei lying closely on the side of the fibre and bulging out 

 the sheath. In their ultimate ramifications they tend to form close networks or 

 plexuses and appear to lose the last traces of a sheath. 



The medullated nerves are bound together by connective tissue (endoneurium) 

 into small bundles, which ?re again united by tougher connective tissue into larger 

 nerve-trunks. These fibres as a rule branch only when in close proximity to their 

 destination, and then the branching always occurs at a node of Ranvier. 



Fro. 102. Non-medullated nerve fibres. (ScnlFEB.) 



As to the functions of the myelin sheath in the medullated nerve fibre very little 

 is known. It does not make its appearance until the axis cylinder is formed, and is 

 apparently derived from a series of cells which grow out from the spongioblasts of the 

 central nervous system and form a chain surrounding the out-growing axons. In the 

 regeneration of a nerve fibre after section the myelin sheath appears later than the 

 axon in the peripheral part of the nerve. It has been supposed by some to act as a sort 

 of insulator ensuring isolated conduction within any given nerve fibre. We have how- 

 ever no proof that equally isolated conduction is not possible in the non-medullated 

 fibres of the visceral system, although it is certainly true that a finer ordering of move- 

 ments is required in the skeletal muscles than in the visceral unstriated muscles. More- 

 over in the central nervous system the main tracts cannot be shown to be functional 

 before the date a't which they acquire their medullary sheaths, suggesting that pre- 

 viously 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 outgrowth. 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. 



