5~-CeH Body 



Medulla 



{ Node ofltawzler 



Neurtienma 



THE NERVOUS SYSTEM AND ORGANS OF SENSE 401 



or organ of sensation. The axis cylinder process 

 forms the pathway over which nervous impulses 

 travel to and from the nerve centers. 



A nerve consists of a bundle of such tiny axis 

 cylinder processes, bound together by a connec- 

 tive tissue. As a nerve ganglia is a center of 

 activity in the nervous system, so a nerve cell 

 is a center of activity which may send an impulse 

 over this thin strand of protoplasm (the axis 

 cylinder process) prolonged into a nerve fiber 

 many hundreds of thousands of times the length 

 of the cell. Some nerve cells in the human body, 

 although visible only under the compound micro- 

 scope, give rise to axis cylinder processes several 

 feet in length. 



Because some nerve fibers originate in organs 

 that receive sensations and send those sensations 

 to the central nervous system, they are called 

 sensory nerves. Other axis cylinder processes 

 originate in the central nervous system and pass 

 outward as nerve fibers; such nerves produce 

 movement of muscles and are called motor nerves. 



The Brain of Man. In man, as in the frog, 

 the central nervous system consists of a brain 

 and spinal cord inclosed in a bony case with the 

 nerves leaving it. From the brain, twelve pairs of nerves are given off ; 



Cerebrum 



Nerve-end* 



Diagram of a neuron or 

 nerve unit. 



Cerebellum 



Medulla 



The brain, with parts separated to show each clearly. 

 HUNT. ES. BIO. 26 



