168 



GENERAL ZOOLOGY 



N. 



_M 



.AX 



fibers. An afferent or sensory nerve is one composed of dendritic 

 fibers and one that conducts impulses from the surface of the 

 body, or from some outlying point, to a centrally located nerve 

 center, such as the brain or spinal cord. An efferent or motor 



nerve is made up of axons and 

 transmits impulses in the opposite 

 directions. Efferent nerves are 

 called motor nerves because their 

 stimulation commonly produces 

 muscular contraction, which in 

 turn produces movement of the 

 body as a whole or in part. Ex- 

 perimentally the direction of the 

 impulse through a nerve may be 

 changed, though normally the direc- 

 tion is from dendron to cell body 

 to axon. 



A ganglion is a mass of nervous 

 tissue consisting largely of nerve 

 cell bodies. In higher forms, nerve 

 cell bodies are confined to ganglia, 

 special nervous epithelia, and nerve 

 centers such as the brain and spinal 

 cord. Different levels of the brain 

 and spinal cord are connected by 

 nerve tracts, composed of nerve 

 fibers, which are comparable to the 

 peripheral nerves connecting the 



Fig. 110. — Diagram of a verte- .,, 



brate motor neuron with a medul- central nervous system with vanous 



lated axon, a, axon; ax, axis p ar tg of the body. Many of the 

 cylinder; b, cell body; d, dendrons; , , , . 



E , motor end organ in contact with connecting pathways of the brain 



muscle cells; M, myelin sheath; n, an( J S p ma l cord Consist of neurons 

 nucleus of sheath cell; s, one of the ... , , 



cells forming a sheath enclosing the lying entirely within the central 

 myelin. nervous system. 



A medullated nerve fiber is one provided with a sheath of white 

 fatty material known as myelin. The axis cylinders of the spinal 

 nerves of vertebrates in addition are provided with a neurilemma 

 composed of delicate sheath cells surrounding the myelin (Fig. 

 110). Medullated fibers of the brain and spinal cord do not have 

 sheath cells. Nerve fibers of the sympathetic system, to be 



