CHAPTER VIII 



THE NERVOUS SYSTEM 



THE STRUCTURE OF NERVOUS TISSUE 



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. It may be compared to the cable, 

 composed of many thousand wires, of a telephone system. 

 When studied with the aid of a microscope the nerve-fibers are 

 found to be very delicate structures. They are of two kinds. 

 The medullated nerve-fibers are fine filaments surrounded by a 

 thick,, white medullary sheath. This sheath is not continuous, 

 but is interrupted at regular intervals so as to expose the fiber. 

 The neurilemma is a thin sheath directly surrounding the nerve- 

 fiber and its medullary sheath. Medullated nerve-fibers are 

 found in the nerves of the cerebrospinal system. The non- 

 medullated nerve-fibers have only the neurilemma for a covering. 

 They occur principally in the sympathetic nerve-trunks and 

 plexuses. 



The nerve-cell, or neuron, is the unit from which the nervous 

 system is built. It consists of a large, irregular, nucleated cell- 

 body that gives origin to one or more fine tapering processes. 

 These nerve-processes are of two kinds— the axon, or axis- 

 cylinder process, which becomes the nerve-fiber previously 

 described; dendrites, or protoplasmic processes, which branch out 

 to bring the nerve-cell into definite relationship with others 

 (see Fig. 2). The axons of nerves in some organs are ex- 

 tremely short, while in the limbs they reach from cells located in the 

 spinal cord to the foot. The cell-bodies he in the gray matter 

 of the spinal cord, brain, and ganglia. 



Nerve ganglia are hard, grey masses found on the dorsal roots 

 of all spinal nerves and along the course of some other nerves. 

 They are composed of groups of cell-bodies. 



Nerve-endings differ materially in arrangement depending 

 on location. Examples of sensory nerve endings are the retina 

 of the eye and the auditory nerve of the ear. The muzzle con- 

 tains highly developed nerve terminations. 



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