404 



THE NBEVOUS SYSTEM 



^9-Jh?^*3olar Plexus 



example, a motor fiber transmits an impulse -n'hich may 

 retard or increase a motion, wliile a sensory fiber transmits 

 impulses which may increase or decrease sensation. Those 

 fibers, either motor or sensory, which excite to greater 

 motion or sensation, are called excitatory. Those which 



decrease motion or sensation 

 are called inhibitorj'. This 

 inhibition or excitation is 

 supposed to be determined, 

 not by the nature of the im- 

 pulse, but by the tissue in 

 which the fiber ends. 



The plexus. — The fact just 

 noted, that the spinal nerves 

 are composed of bimdles of 

 fibers, explainis how it is pos- 

 sible for them to split up into 

 smaller nerves and viltimately 

 into separate fibers, just as we 

 may um-avel the strands of a 

 rope. In many regions of the 

 body some of these unravelled 

 nerve fibers from several 

 spinal or ganglionic nerves 

 interlace to form a meshwork, and such networlcs are called 

 plexuses. In this fusion, new combinations of fibers are 

 often made, and nerves extending from a plexus may con- 

 tain fibers from several different spinal or ganglionic 

 nerves. E^'idently the stimulation of such a nerve will 

 result in messages to several different parts of the central 

 system, and vice versa. For example, the nerves supply- 

 ing the legs arise from large plexuses, and are thus supplied 



^^Abdominal 

 AortcL 



— ^^.Sympathetic 

 Cluzin 

 H. Sympathetic 

 Chain 



'^liMypogastria 

 Plexus 



'jj-JHoo a. 



Fig. 191 — The solar and hypogastric 

 sympathetic plexuses. 



