CONTROL OF THE CIRCULATION. 



may have, they seem to have the power of retarding, or 

 stopping altogether, the beat of the heart ; and stimulation 

 of the vagus nerves may make the heart pause in a relaxed 

 condition. Other nerves may quicken the heart beat, but 

 the vagi are regarded as a break on the heart's action. 



Inhibition. This is a case of inhibition. It is well 

 known that a severe blow over the stomach may cause one 

 to faint by stopping the heart. This 

 is due to reflex inhibition of the heart. 

 The blow sends a nerve impulse by 

 fibers of the sympathetic system to 

 the center in the spinal bulb, and 

 thence an impulse is taken by the 

 vagus nerves to stop the heart. 



Vaso-constrictor Nerves. In an 

 experiment with the rabbit's 

 ear it has been shown that 

 stimulating the sympathetic 

 nerve in the neck causes the 

 ear to become pale. This is 

 due to the constriction of the 

 arteries of the ear, because 

 the nerves have made the 

 muscle fibers of these arteries 

 shorten. Such nerve fibers 

 are called constrictors, or vaso-con- 

 strictors. They run in the sympa- 

 thetic nerve, but have their origin 

 and center in the spinal bulb. 



Vaso-dilator Nerves. Other fibers 



may cause the opposite effect, namely, dilation, and are 

 therefore called vaso-dilators. Examples of these may be 



Lungs- 



Heart 



Liver 



Fig. 32. Diagram of Vagus 

 Nerve. 



