106 NORMAL HISTOLOGY. 



nerve-tissues into those of the cerebro-spinal and 

 the sympathetic systems ; the medullated belonging 

 to the former, the non-medullated to the latter. 



a. Medullated Nerve-fibres. 



If we disregard for the moment the structure of 

 these nerves at their point of origin in the nerve- 

 centres, and at their termination in the periphery, 

 and study their structure as it is seen in the con- 

 tinuity of any of the larger or smaller nerves, we 

 find that the individual fibres present three distinct 

 structural elements; I, the axis cylinder; 2, the 

 medullary sheatli ; 3, the neuri lemma. 



1. Running through the axis of the fibre is a 

 cylindrical, with high powers, delicately longitudi- 

 nally striated structure the axis cylinder. This 

 is believed to be the essential nerve-element of 

 the fibre the process of the nerve-cell, from which 

 it passes without break of continuity to the periph- 

 ery ; and it is probable that the longitudinal stri- 

 ations are the expression of its composition from 

 still finer primitive fibrils, which, as we shall see 

 when we study the nerve-cells, seem to be continued 

 on into the cell-body itself, within the nerve-centres. 



2. Closely surrounding the axis cylinder is a tube 

 or sheath of varying thickness the medullary sheath 

 composed of a white, semi-fluid, translucent, 

 strongly refractive substance, called myelin, which 

 undergoes rapid changes after death, or on removal 



