INTEGRATION OF ACTIVITIES 



143 



emerge, between the vertebrae, pairs of spinal nerves (31 of these in man). 

 Each of the spinal nerves arises from the cord by two roots, a dorsal 

 and a ventral, which join in a single nerve trunk a short distance from 

 the cord (Fig. 121). The dorsal root includes a ganglion which contains 



a host of nerve-cell bodies. The 

 relation of the peripheral to the 

 central system in the frog is illus- 

 trated in Fig. 117. 



A special part of the peripheral 

 system is known as the autonomic 



vm 



T^\^J XandX 



cer 



Fig. 117 Fig. 118 



Fig. 117. — Nervous system of frog, ventral view. I-X, cranial nerves; a, autonomic 

 system; cer, cerebrum; n, nasal sac; op, optic lobe; spc, spinal cord. (After Wiedersheim.) 



Fig. 118. — Diagram of a typical neuron, ax, axon; d, dendrite; ms, medullary sheath; 

 mu, muscle; n, node; ne, nerve endings; nu, nucleus of cell of neurilemma. 



nervous system because of its control, entirely free of the will, of many 

 vital functions. It consists visibly of a pair of ganglionated cords on 

 either side of the vertebral column, lying exposed in the body cavity. 

 The ganglia are connected with the spinal cord by nerve fibers passing 

 through the ventral roots of spinal nerves. In the extreme anterior and 

 posterior parts of the autonomic system, however, there are nerve fibers 

 which pass directly from the central nervous system to the organs con- 

 trolled without connections in centrally placed ganglia. 



Unit of Structure of Nervous System. — The unit of structure of the 

 nervous system is the neuron. The neuron is a cell possessing a number 



