34 STRUCTURE OF NERVE FIBER. 



An individual nerve fiber is too small to be seen by the 

 naked eye, being only about one two-thousandth of an inch 

 in diameter. There are two kinds, white and gray. A White 

 Nerve Fiber consists of the following parts : 



1. A central strand, or core, of semi-transparent, grayish- 

 looking material, called the Axis-cylinder. This is the essen- 

 tial part of the nerve fiber, and does the work of the nerve, 

 which is to convey Nerve Currents, or Nerve Impulses. 



2. Around the axis-cylinder is a layer of white, oily mate- 

 rial, known as the Medullary Sheath. 



Nerve Fiber Sheath 



Axis Cylinder 



Medullary Sheath 

 Fig. 11. Structure of a Nerve Fiber. (Diagram.) 



3. Outside this is a thin, transparent sheath of connective 

 tissue, essentially like the muscle-fiber sheath, the Nerve-Fiber 

 Sheath. 



The sole function of the nerve fiber is to convey nerve im- 

 pulses. 



Gray Nerve Fibers have no medullary sheath, but consist 

 simply of the axis-cylinder and the nerve-fiber sheath. They 

 are found principally in the sympathetic nerves. 



Cross-section of the Spinal Cord. If a thin slice of 

 the spinal cord be made as shown in Fig. 12, it will be seen 

 that the central portion is of a darker color than the outer 

 part. 



The central part is known as the Gray Matter, in distinction 

 from the rest, which is called the White Matter. The white 



