Chap. 16 CONDUCTION AND COORDINATION NERVOUS SYSTEM 293 



Spinal Nerves. The spinal nerves occur in pairs, one on cither side of the 

 nerve cord. In the higher vertebrates, including man, each trunk branches 

 from the nerve cord by two roots; the dorsal one contains nerve cell fibers 

 that carry sensory messages into the cord, and the ventral one contains those 

 carrying messages to the muscles and glands (Figs. 16.12, 16.14). The cell 

 bodies of fibers in the dorsal root are contained in its ganglion, but those of 

 the ventral root are always in the gray matter of the cord. Soon after the 

 sensory nerve fibers have entered the spinal cord they may come in contact 

 with adjustor neurons and so take part in reflex responses or they may 

 participate in carrying messages to the brain. 



White matter 



Gray matter 



Dorsal root 

 (sensory) 



Synapse 



Centra 

 cana 



Communicotinq ram 



Sympathetic ganglion 



Intestine 

 (Cross section) 



Skin 

 (receptor) 



Skin 

 receptor) 



untary) 

 fector) 



Sensory nerve cell 

 Motor nerve cell 

 Adjustor nerve cell 



O- 



-f (somotic) 

 - (somatic) 

 -< (somatic) 







(visceral) 

 (visceral ) 



Fig. 16.14. Diagram showing the close association of the nerve cells (autonomic 

 or sympathetic) that control involuntary muscles of the intestine and those that 

 control the voluntary or somatic muscles. Only a few are drawn out of the great 

 numbers of cell bodies and fibers in the cross section of the cord. Cell fibers 

 carrying impulses from sensory stimuli in skin or muscle enter the cord through 

 a sensory root; cells bearing impulses that result in contraction of muscle leave 

 the cord by a motor root. Cell fibers of the autonomic nerve form part of this 

 motor root. All of the thousands of fibers are one way passages. (Modified from 

 Neal and Rand: Comparative Anatomy. Philadelphia, The Blakiston Co., 1936.) 



