INHIBITION OF THE HE AST. 



This is a case of Inhibition. It is well known that a 

 severe blow over the stomach may cause one to faint by stop- 

 ping 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 pneumogastric 

 nerves to stop the heart. 



In the experiment with the rabbit's 

 ear we saw that stimulating the sympa- 

 thetic nerve caused the ear to become 

 pale. This, we understand, is due to 

 the constriction of the arte- 

 ries of the ear, because the 

 nerves have made the muscle 

 fibers of these arteries short- 

 en. Such nerve fibers are 

 called Constrictors, or Vaso- 

 constrictors. They run in 

 the sympathetic nerve, but 

 have their origin and center 

 in the spinal bulb. Other 

 fibers may cause the opposite effect, 

 namely, dilation, and are therefore 

 called Vaso-dilators. Examples of 

 these may be found running to the 

 arteries of the limbs. We have seen 

 that when the muscles of any organ, 



say the legs, act, they need a greater supply of blood. Now, 

 at the same time that nerve impulses are sent to the muscles 

 of the legs to make the muscles shorten, impulses are sent 

 along other fibers of the same nerves to make the arteries 

 dilate, and allow more blood to flow to these muscles. 



Lu rigs 



Heart 



Liver 



Stomach 



Fig. 35. Diagrarr <// Pneu- 

 mogastric Nerve. 



