THE STRUCTURE OF NERVE FIBRES 281 



and staining black with osmjc acid, supported in the interstices 

 of a network formed of a horny ubstance known as neurokeratin. 

 The medullary sheath is surrounded by a structureless membrane, 

 the primitive sheath or neurilemma. At regular intervals a break 

 occurs in the medullary sheath, the neurilemma coming in close 

 contact with the axis cylinder. This 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 dis- 

 tributed 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 collection of ganglion-cells of the sympathetic chain 



FIG. 103. Non-medullated nerve fibres. (SCHAFER.) 



or peripheral ganglia, the impulses being carried on to their destina- 

 tion by a fresh relay of non- medullated nerve fibres. 



The non-medullated fibres (Fig. 103) 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 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 branch- 

 ing 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 re- 

 generation 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 con- 

 duction 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 

 case of the visceral unstriated muscles. Moreover in the central 



