THE SYMPATHETIC SYSTEM 



229 



a given irritation is invariably followed by the correct 

 protective movement. 



Branches of spinal nerves. In addition to the sys- 

 tem of spinal motor and sensory nerves, there is another 

 system of branches from the spinal nerves which have 

 charge of. the parts of the body not under voluntary 

 control, that is the viscera. These branches leave the 

 spinal nerves near the spinal cord and pass into the 



Spinal cord 



Motor (secretory) nerve to 

 sweat gland 



Artery 



Sweat gland 



Motor nerve cells 

 Intermediate nerve cells 

 Motor (vasodilator) nerves to blood vessels of skin 



FIG. 123. Diagram showing a part of the reflex mechanism by which heat when 

 applied to the skin results in the dilatation of the blood-vessels in the skin and 

 in the secretion of perspiration. 



chest and abdominal cavities. Here they join a chain of 

 ganglia which lie close to the back wall of these cavities. 

 From the ganglia issue many nerve branches which pass 

 to the heart and blood-vessels, the lungs, stomach, in- 

 testines and other organs of the chest and abdomen. 



Sympathetic system. This system of nerves is known 

 as the sympathetic system, because it controls the activi- 

 ties of the viscera automatically through its sensitive- 

 ness to their needs and the needs of the body. For 



