CHAPTER VIII 



THE NERVOUS SYSTEM 



THE STRUCTURE OF NERVOUS TISSXJE 



A nerve, as seen in a piece of flesh, consists of an elongated 

 cord made up of bundles of nerve-fibers and having the property 

 of transmitting impulses. When studied with the aid of a micro- 

 scope the nerve-fibers are found to be very delicate structures. 

 They are of two kinds. The tnedullated nerve-fibers are fine fila- 

 ments surrounded by a thick, white medullary sheath. This 

 sheath is not continuous, but is interrupted at regular intervals so 

 as to expose the neurilemma, a thin sheath directly surrounding 

 the nerve-fiber. These meduUated nerve-fibers are found in the 

 nerves of the cerebrospinal system. The non-tnedullated nerve- 

 fibers have only the neurilemma for a covering. They occur prin- 

 cipally in the sympathetic nerve-trunks and plexuses. The nerve- 

 cell is the unit of the nervous system. It is composed of a cell- 

 body and one or more elongated processes. The nerve processes 

 may be looked upon as outgrowths of the cell-body and are of two 

 kinds — the axon or axis-cylinder process which becomes the 

 nerve-fiber previously described, and dendrites or protoplasmic 

 processes which branch out to bring the nerve-cell into definite 

 relationship with others (see Fig. 1). The axons are extremely 

 short in some organs, while in the limbs they reach from cells lo- 

 cated in the spinal cord to the foot. 



There are two distinctly different kinds of nerve substance — 

 the white matter which is soft and contains about 70 per cent, of 

 water, and the gray matter of brown color and softer consistency 

 than the white substance containing over 80 per cent, of water and 

 about 10 per cent, of proteins. 



Ganglia are hard, gray masses found on the dorsal roots of 

 spinal nerves and along the course of other nerves. They are 

 composed largely of groups of cell-bodies. 



Nerves terminate in the muscles, skin, and joints. The mus- 

 cular branches are motor in function, the others sensory, but con- 

 tain vasomotor fibers which control the caliber of the blood-vessels. 



119 



