252 PHYSIOLOGY 



break is the node of Ranvier, the intervening portions of medullated nerve 

 being the internodes. In each internode, lying closely under the neurilemma, 

 is an oval nucleus embedded in a little granular protoplasm. The medullated 

 nerve fibres vary considerably in diameter, the largest fibres being distributed 

 to the muscles and skin, the smallest carrying impulses from the central 

 nervous system to the viscera. The latter all come to an end in some collec- 

 tion of ganglion- cells of the sympathetic chain or peripheral ganglia, the 

 impulses being carried on to their destination by a fresh relay of non- 

 medullated nerve fibres. 



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 



FIG. 102. Non-medullated nerve fibres. (SCHAFER.) 



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 (endo- 

 neurium) into small bundles, which are 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. 



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, however, 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 movements is required in the skeletal muscles than in the 

 visceral unstriated muscles. Moreover in the central nervous system the 

 main tracts cannot be shown to be functional before the date at which they 

 acquire their medullary sheaths, suggesting 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 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. 



