NERVE FIBERS 2 17 



description of them as separate parts is warranted for the sake of 

 convenience and by differences in their general characteristics. 



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

 stances, of generating nerve force. As a rule they are stimu- 

 lated to generate this force by the reception of an impression 

 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 impulses, and 

 they usually receive these impressions and impulses 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 directly stimulated. The nerves of 

 special sense are insensible to direct stimulation. 



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

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

 non-medullated fibers possess the conducting elements alone, 

 while the medullated possess certain accessory anatomical 

 elements. 



(A) Each medullated fiber has (i) an external enveloping 

 membrane called the neurilemma, or the primitive nerve sheath, 

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

 what elastic, and presents oval nuclei with their long diameter 

 corresponding to the direction of the fiber . This sheath is want- 

 ing over the medullated fibers in the white substance of the brain 

 and spinal cord. 



