NERVE FIBERS 217 



warranted for the sake of convenience and by differences in 

 their general characteristics. 



The nerve cells are the only organs capable, under any cir- 

 cumstances, of generating nerve force. As a rule they are 

 stimulated to generate this force by the reception of an im- 

 pression through the nerve fiber, but they may in some cases 

 be directly excited by mechanical, electrical or chemical 

 means. They also frequently act as conductors, as will be 

 seen later. 



Under no circumstances can nerve fibers generate force. 

 Their office is exclusively to conduct impressions and im- 

 pulses, and they usually receive these impressions and im- 

 pulses at their terminal extremities* in the case of afferent 

 nerves, and from the centers in the case of efferent nerves ; 

 but in many instances they may be stimulated in any part of 

 their course. Some fibers are incapable of being thus di- 

 rectly stimulated. The nerves of special sense are insensi- 

 ble to direct stimulation. 



Nerve Fibers. Nerve fibers are of two kinds: (A) white 

 or medullated fibers and (B) gray or non-medullatcd fibers. 

 The non-medullated fibers possess the conducting elements 

 alone, while the medullated possess certain accessory ana- 

 tomical elements. 



(A) Each medullated fiber has (i) an external envelop- 

 ing membrane called the ncurilemma, or the primitive nerve 

 sheath, or the sheath of Schwann; (2) an intermediate sub- 

 stance known as the myeline sheath, or the white substance 

 of Schwann, or the medullary substance; (3) a central 

 fiber, the true conducting element, which usually goes under 

 the name of the axis cylinder, or axone. 



The sheath of Schwann is analogous to the sarcolemma 

 of muscle fibers. It is a structureless protective membrane, 

 somewhat elastic, and presents oval nuclei with their long 

 diameter corresponding to the direction of the fiber. This 

 sheath is wanting over the medullated fibers in the white sub- 

 stance of the brain and spinal cord, 



