28 PHYSIOLOGY. 



3. The Nerve Fiber Sheath is a thin, transparent outer 

 sheath of connective tissue. 



Function of Nerve Fibers. The sole function of the 

 nerve fiber is to convey nerve impulses. The nerve im- 

 pulse passes along the axis cylinder as an electric current 

 passes along an insulated wire. 



Nerve Fiber Sheath 



Axis Cylinder 



Medullary Sheath 

 Fig- 10. Structure of a Nerve Fiber. 



Gray Nerve Fibers. In the sympathetic nerves there 

 are many fibers which have no medullary sheath, but con- 

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

 These are called gray nerve fibers. 



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

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

 seen that the central part is darker in 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 matter of the nervous system is made up of 

 nerve fibers whose structure and use we have just con- 

 sidered. But the gray matter has a different structure and 

 a different function. Instead of being made up mainly of 

 fibers it is composed of cells, one of the forms of which is 

 represented in Fig. 12. Some of the branches of these 

 cells are continued, and become the axis cylinders of nerves, 

 and it is believed that every nerve fiber begins as a branch 

 of some nerve cell. One of the best places to see these 

 nerve cells is in the gray matter of the spinal cord, near 



