ANATOMICAL DESCRIPTION 



277 



The central thread of a nerve fiber may have two inclos- 

 ing sheaths. A layer of white, oily matter immediately 

 surrounding the axis cylinder is called 

 the medullary sheath, or 

 sometimes the white sub- 

 stance of Schwann. Out- 

 side of this is a coat of 

 thin, elastic membrane 

 called the primitive sheath, 

 or neurilemma. The lat- 

 ter covers its fiber from 

 end to end, but the med- 

 ullary sheath is broken 

 at frequent intervals, and 

 between the breaks the 

 microscope shows along 

 the course of the axis lit- 

 tle nuclei buried in min- 

 ute masses of protoplasm. 

 Fig. 123. Nerve I n some nerve fibers 

 cells (pyramidal) the medullary sheath is 

 of the cortex of the . 



cerebrum. wanting, leaving only the Fig. 124. Por- 



Ax axis cylinder proo neurilemma inclosing the duSated^erve 



axis cylinder. These are fiber. 



called nonmedullated nerve fibers. The medullary sheath 

 is brilliant, shining white in color, and gives to the nerve 

 its characteristic white appearance. The nonmedullated 

 fibers, therefore, are gray. 



Nerve fibers may be very short or they may be many 

 feet in length. Every nerve fiber originates in a nerve 

 cell, but there are several ways in which it may end, as 

 has already been shown. 



411. A Nerve is a bundle of nerve fibers bound together 



l )_Nodeof 

 Ranvier 



Neurilemma 



(--Nucleus 



Neuraxon 

 " 'or Axis 

 Cylinder 



Medullary 

 ' 'Sheath 



