THE NEKVE FIBER 



137 



lio 



8*3 



m** 



and its nucleus with a minute amount of undifferentiated protoplasm is, 

 according to this hypothesis, considered to remain as a permanent part 

 of the neurolemma. 



2. Medullated Nerve Fibers without a Neurolemma. This type 

 of nerve fiber composes the white matter of the central nervous system. 

 The axis cylinder does not, of course, differ in the least from those of 

 the previous variety and will need no further description. 



The medullary sheath 

 also is similar in its finer 

 structure to that of the 

 previous type, but since 

 no neurolemma is pres- 

 ent, these fibers possess 

 no nodes of Ranvier. The 

 medullary sheath of the 

 fibers found in the white 

 matter of the brain and 

 spinal cord, is therefore 

 uninterrupted. Its sur- 

 face is in direct contact 

 with the neuroglia net- 

 work, which forms the 

 supporting tissue of these 

 organs, the innermost 

 layer being condensed 

 into a membrane which 

 simulates a neurolemma. 

 These fibers are accom- 

 panied by sheath cells 



(Hardesty), homologues of the neurolemma cells of other fibers, which 

 aid in the formation and maintenance of the myelin. 



3. Non-medullated Nerve Fibers with a Neurolemma (Sympar 

 thetic Nerve Fibers, Remak's Fibers}. The most of the fibers of the 

 sympathetic division are of this type. The axis cylinder does not differ 

 from that of the previous types. The medullary sheath is entirely ab- 

 sent or, at most, only slightly developed in these fibers. The neuro- 

 lemma is perhaps incomplete at times, but exhibits frequent nuclei 

 along the course of the fiber. The neurolemma nuclei appear to be em- 

 bedded in the superficial portion of the axis cylinder. Fibers of this 

 type are of quite frequent occurrence also in the cerebral (cephalic; 



m 



*w 



'.jf' 



* 



FIG. 150. CROSS-SECTION OF THE TRUNK OF THE 

 HUMAN VAGUS NERVE, SOME DISTANCE BELOW 

 THE NODOSE GANGLION, SHOWING MEDULLATED 

 AND NON-MEDULLATED FIBERS. 



Pyridin-silver. X 680. (After Ranson.) 



