CHAPTER XIX 



THE PERIPHERAL NERVES AND THE SYMPATHETIC 



SYSTEM 



1. THE SPINAL NERVES 



THE spinal nerves leave the spinal cord by the anterior 

 and posterior roots. 



The anterior roots are motor, the posterior chiefly sensory 

 (Bell's law), but the latter also contain a few motor nerves 

 for the muscles of the intestines. 



The nerve fibres innervating a muscle do not all lie in the 

 same motor root, but a muscle is supplied with motor fibres 

 from several anterior roots. These fibres join each other 

 (plexus) and then proceed in a common trunk to the muscle. 



The anterior roots contain fibres whose simultaneous stimula- 

 tion, calls forth movements of entire muscle groups which re- 

 semble certain coordinated movements frequently executed in life. 

 For example, stimulation of the first dorsal root in a monkey re- 

 sults in the movements of the arm similar to those made in pluck- 

 ing fruit ; stimulation of the seventh cervical calls forth movements 

 of the arms similar to those made in climbing ; by stimulation of 

 the sixth cervical the hand is carried to the mouth. Perhaps the 

 cells from which these nerves originate lie together in special cell- 

 groups in the spinal cord, which may be regarded as coordinated 

 centres. From these centres the nerve fibres accompany each 

 other to the plexus. 



The functions of the individual spinal nerves can be 

 learned from their anatomical connections. 



2. CRANIAL NERVES 



I. The olfactory nerve is the nerve of smell. The olfac- 

 tory bulb is the sub-cortical centre of this nerve ; in it cells 

 are interposed in the tract. 



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