602 PHYSIOLOGY. 



branch to form arborizations to come into contact with nerve-den- 

 drites. 



THE NERVE-FIBERS. Every nerve-fiber is a process of a nerve- 

 cell. It is the neuraxon of some particular cell. It is the medium 

 which conducts impulses to or from the tissues and organs, on the 

 one hand, and the nerve-centers, on the other. In the majority of 

 cells the neuraxon acquires a sheath to be thus converted into a 

 medullated nerve-fiber. Thus, there are two kinds of nerve-fibers: 

 medullated, or those with myelin ; and nonmedullated, or those without 

 myelin. 



Medullated Fibers in the fresh condition are bright, glistening 

 cylinders showing a dark, double contour. The essential part of the 

 fiber is the axis-cylinder. This is a soft, transparent rod, or thread, 

 which runs from one end of the fiber to the other. It does not 

 anastomose with its neighbors, and in the average nerve is about Vi2oo 

 inch in diameter. After the employment of certain reagents the 

 axis-cylinder shows itself to be composed of very fine, homogeneous 

 or more or less beaded fibrilla?. The latter are the elementary, or 

 primitive fibrillce. They are held together by a small amount of a 

 faintly granular, interstitial substance. The thickness of the axis- 

 cylinder is in direct proportion to the thickness of the whole nerve- 

 fiber. The axis-cylinder is enveloped in its own, more or less elastic, 

 hyaline sheath. 



The axis-cylinder is not regularly cylindrical, but is slightly nar- 

 rowed in places. Under the influence of silver nitrate applied to its 

 surface there appear alternate obscure and clear transverse striae. 

 They are the so-called lines of Frommann. 



Myelin. Surrounding the axis-cylinder is the myelin, medullary 

 slicatli, or the white substance of Scliwann. It is a layer of fatty sub- 

 stance, strongly refracting, and of homogeneous aspect. It is colored 

 black by osniic acid. It is the myelin which gives to the nerve its 

 double contour. It is composed of a network of fibrils of a chemical 

 substance called neuroJceratin, which incloses the semi-fluid, fatty sub- 

 stance. The latter contains, among other substances, a complex, 

 phosphorized fat. 



The sheath of myelin envelops the axis-cylinder everywhere, ex- 

 cept at its termination and at the nodes of Ranvier. 



In its arrangement the myelin is imbricated in the fashion of 

 tiles on a roof by reason of a series of segments one above the other. 

 They are separated one from the other by clear lines. The lines are 



