40 Physiology. 



root, the other nearer the ventral surface, the ventral root. 

 These two roots soon unite to form one spinal nerve. 



Structure of Nerves. If we trace a nerve outward, we 

 find that it is continually subdividing. This division con- 

 tinues until the branches are too small to be seen by the 

 naked eye. Microscopic examination shows that a nerve 

 is made up of a great number of fibers bound together in 

 a common sheath of connective tissue, as is the case with 

 muscle. When the nerve divides, there is ordinarily no 

 true branching or forking, but certain of the fibers simply 

 separate from the rest, as in the separation of the fibers in 

 floss silk. 



Nerve Fiber Sheath 





I Axis Cylinder 



Medullary Sheath 

 Fig. 24. Structure of a Nerve Fiber. 



Structure of a Nerve Fiber. A single nerve fiber is too 

 small to be seen by the naked eye, being only about one 

 two-thousandth of an inch in diameter. It consists of the 

 following parts : 



1. The axis cylinder, a central strand, or core, of semi- 

 transparent, gray material. 



2. The medullary sheath is a layer of white, Oily ma- 

 terial around the axis cylinder. 



3. The nerve fiber sheath is a thin, transparent outer 

 sheath of connective tissue. 



Function of Nerve Fibers. The only 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. 



