NERVOUS SYSTEM 389 



importance to and from the brain ; and third, it controls 

 to a great extent the digestive and the circulatory 

 systems. 



Sympathetic nervous system. A chain of ganglia on 

 each side of the spinal column together with three 

 large ganglia and a network of nerves, situated in the 

 middle line of the body, form what is often called the 

 sympathetic nervous system. It is not a distinct system 

 at all, but is closely connected with the brain and the 

 spinal cord. Nerves from these centers pass to many 

 of the internal organs, to the skin, to the glands, and 

 to the muscles. This system also relieves the brain, for 

 through its centers many of the involuntary activities 

 are controlled. One of these centers, located just 

 behind the pit of the stomach, is the so-called "solar 

 plexus." A blow in the region of the stomach paralyzes 

 many parts, and may even result in instant death. 



Action of nervous system. Even the simplest motions 

 which we perform involve the action of many nerves 

 and nerve centers. If the finger is accidentally pricked 

 by a pin, a touch organ in the skin is stimulated. From 

 this currents, or impulses, are sent over several nerve 

 fibers through the dorsal root into the spinal cord. 

 As a result of changes which this produces in cells in 

 the spinal cord, currents are sent to motor cells in the 

 cord. From these motor cells currents are sent over 

 fibers in the motor root to a muscle in the arm. This 

 causes a contraction of the muscle which produces a 

 motion, the response, as a result of which the hand is 



