SPINAL NEBVE TRUNK AND FIBER 



403 



_ Node of 

 Banvier 



Neuraxon 

 '"or Axis 

 Cylinder 



Medullaru 

 "'Shaath 



of Ranvier." Outside this myelin sheath is a thin mem 

 brane called the neurilemma, and comparable to the sar- 

 colemma of the muscle fiber. Lying under 

 the neurilemma are found nuclei, one for 

 each segment of the medullary layer. These 

 two outer layers of the fiber do not conduct 

 impulses, and while their function has not 

 been absolutely determined, they may be 

 compared to the insulation which surrounds 

 an electric wire. The axis, evidently, is 

 the wire. In some nerve fibers the medul- 

 lary layer is lacking, the axis being sur- 

 rounded by the neurilemma only. 



All nerve fibers originate either in the 

 nervous system or in the sympathetic 

 ganglia. Those from the former are all 

 meduUated; some of those of the latter are 

 non-medullated. All fibers admit also of 

 classification as afferent or efferent, since, 

 in a given fiber, the direction in which the 

 impulse travels is always the same. Mixed 

 nerves, therefore, are composite bundles of afferent and 

 efferent fibers. 



The words afferent and efferent simply indicate the di- 

 rection in which the impulse travels. Owing to the fact 

 that efferent fibers all carry impulses which produce motion, 

 the words efferent and motor are equivalent in describing 

 a given fiber, while for the same reason the words afferent 

 and sensory may be interchangeably applied to fibers 

 which convey sensory impulses. 



Finally, the two classes of fibers may be subdivided 

 again on the basis of the effect of their impulses. For 



Fig. 190— Section 

 of a nerve. 



