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TEXTBOOK OF ZOOLOGY 



might be. This system controls the involuntary muscles. The pe- 

 ripheral system includes ten to twelve pairs of cranial nerves from 

 the brain, and ten to thirty-one pairs of spinal nerves in different 

 forms of vertebrates. Each spinal nerve has two roots where it 

 joins the spinal cord. A dorsal root receives fibers from sensory end- 

 ings and therefore conducts impulses toward the cord. This root 

 has a spinal ganglion located on it. The ventral root of each of these 



Fig'. 220. — Cross section of spinal cord and roots of spinal nerves, sliowing a 

 simple reflex circuit. 1, sensoi-y surface of skin ; 2, afferent nerve fiber with S, its 

 cell of origin, located in the spinal ganglion ; i, cut end of spinal nerve ; 5, efferent 

 nerve fiber ; 6, voluntary muscle ; 7, dorsal root of spinal nerve ; 8, ventral root of 

 spinal nerve ; 9, dendrites of motor nerve cell body in gray matter of the cord. 

 (From Zoethout, Textbook of Physiology, published by The C. V. Mosby Company, 

 after Morat.) 



nerves carries fibers extending from the motor cells in the cord to 

 the motor end plates on the voluntary muscle cells. The impulses, 

 therefore, pass from the spinal cord to the muscles over these roots. 

 The reflex arc, which is the simplest kind of a nerve conduction cir- 

 cuit, is set up by the connectives from the sense organ or receptor 

 to the gray matter of the cord and then the return connection from 

 the motor nerve cells over the ventral root to the muscles. In gen- 

 eral, the relationship of parts in regard to function is similar to 

 what has already been seen in the higher nonchordate animals. 



