THE NERVOUS SYSTEM 



227 



sympathetic nerves is formed upon the heart, and about the 

 lungs, the stomach, and the intestines, as well as around 

 the walls of the minute arteries and capillaries. 



A large part of the regulating action of the vital organs 

 is controlled by the influences which thus reach these 

 organs through the 

 sympathetic nerves. 



305. How the Sym- 

 pathetic System acts. 

 It is believed that the 

 sympathetic nerves do 

 not serve as independ- 

 ent nervous centers. 

 The influences which 

 they convey to the 

 great processes that 

 are beyond the control 

 of the will, are derived 

 from the brain or the 

 spinal cord. 



Therefore we may 

 think of these great 

 networks, or plexuses, 

 of nerves, not as a 

 separate nervous 

 system, but as an out- 

 lying part of the cerebro-spinal system. The sympathetic nerves 

 are very slow to act. A blush steals slowly up to the 

 roots of the hair. If we go from the dark into a strong 

 light, we are blinded. The pupil of the eye is too large. 

 An impression is made on the sympathetic nerve, which 

 causes it to contract slowly while we are shading our eyes. 



FIG. 143. 



Showing the distribution of some of the great 

 plexuses of the sympathetic nerve in the 

 lumbar and sacral regions. 



