COORDINATION 



207 



Moreover, nerve impulses are not transmitted by a nerve as a 

 whole, but by one component cell process, a nerve fiber ; that is, by 

 way of a definite cell path through the nerve. The same is equally 

 true of the cord and the brain, which differ from nerves largely in 



Cortex 



BRAIN—/- 



Motor fiber 

 Sensory fiber 



SPINAL CORD 



Dorsal root 



Ventral root 



Synapse 



Motor fiber / ^ 

 SPINAL NERVE 



Sensory nerve 

 /fiber ending in 

 sense cell 



/Motor nerve 



fiber ending 



in muscle 



Fig. 143. — Diagram of the paths of sensory and motor nerve fibers. A 

 reflex arc from sense cell via spinal cord to muscle cell is shown at lower right. 



that they comprise more cell processes and also the cell bodies 

 themselves. In other words, the brain and cord comprise the ele- 

 ments of both ganglia and nerves. 



A mixed nerve conducts impulses both to and from the central 

 organ because it contains both sensory and motor cell paths, or 

 fibers. All peripheral nerves are primarily mixed nerves, because 

 typically they arise by two roots from the central organ ; the dorsal 

 root containing only sensory (afferent) fibers and the ventral 



