AXIS-CYLINDER. 



207 



FIG. 78. 



Nerve-fibres. 



Nerve-fibres are regarded as processes of nerve-cells, being 

 the continuations of neurites, enveloped 

 by certain sheaths. They connect nerve- 

 cells with nerve-terminals, and their func- 

 tion is the conduction of nervous impulses. 

 Nerve-fibres are situated both within and 

 without the cerebro-spinal centres, with 

 certain differences in their envelopes in the 

 two situations. In their passage through 

 the body the fibres are united by sustentac- 

 ular tissue into bundles, the nerves or nerve- 

 trunks. Nerve-fibres are of two kinds, 

 medullated and non-medullated. 



Medullated nerve-fibres (Fig. 78), also 

 called myelinic or white fibres, make up the 

 chief part of the cerebro-spinal nerves and 

 occur also in large numbers in the cerebro- 

 spinal structures. Where most developed 

 they consist of three parts axis-cylinder, 

 medullary sheath, and neurilemma. Within 

 the cerebro spinal centres there is no neuri- 

 lemma, and at the terminations of the fibres 

 both neurilemma and medullary sheath are 

 wanting. 



The axis-cylinder, or axon, the prolonga- 

 tion of a neurite, is a fine continuous fila- 

 ment, uniform in size, occupying the centre 

 or axis of the fibres. In medullated fibres 

 the axis-cylinder does not branch except for 

 the " collateral fibres," and near its distal 

 termination, where it may divide at a node 

 of Ranvier. It exhibits fine longitudinal 

 striations, apparently representing spongio- 

 plastic fibrilla? bound together by cement or 

 protoplasm. It is the essential and conduct- 

 ing portion of the fibres, the medullary sheath and neurilemma 

 serving as coverings for it. [&J. its central end the axis- 

 cylinder emerges as a process from a nerve-cell, and at its 



d- 



Segment of a medul- 

 lated nerve-fibre, a, 

 node of Ranvier; 

 6, neurilemma : c, 

 medullary sheath ; 

 rf, axis-cylinder; e, 

 incisure of Schmidt; 

 /, nucleus of neuri- 

 lemma. 



