278 



ANATOMY AND PHYSIOLOGY 



known as non-medullated nerve fibers, and such are found in in- 

 voluntary muscles and in the walls of internal organs. 



Structures composing a medullated nerve fiber: 



1. The axon or axis-cylinder. 



2. Medulla or myelin (white substance of Schwann). 



3. Neurilemma, a transparent membrane inclosing the myelin (sometimes 

 absent). 



Dendrites 



Nerve cell 



Medullated 

 - fiber 



Nerve cell 



FIG. 172. FIG. 173. 



FIGS. 167, 168. NERVE CELLS. (Brubaker.) 



Structures composing a non-medullated nerve fiber: 



1. The axon or axis-cylinder. 



2. Neurilemma (sometimes absent). 



Medullated nerves are found in voluntary muscles, skin, mucous ana serous 

 membranes, joints and special sense organs. They constitute the main 

 portion of the Cerebro-spinal Division. 



Non-medullated nerves are found in glands, vessels, hollow viscera, and 

 muscle fibers at roots of hairs. They constitute the main portion of the 

 Sympathetic Division. 



The axons or nerve fibers terminate in fine branches, which 

 connect them either with various organs or with the dendrites of 

 other cell bodies, as the case may be. 



For want of more accurate language, we say that impulses are 



