AETBEY CONTEOL 205 



ends in the heart muscle receive this impulse and increase 

 the rate of contraction and expansion of the cells. When 

 another set of nerves is stimulated, they, in turn, cause 

 the contraction to be reduced in rate. These are like 

 telegraph wires connecting stations. One station is the 

 heart where messages are received. The others are found 

 in the brain and in the body tissues, and the messages 

 sent from these latter points control the action of the 

 heart. Of the two kinds of nerves those that decrease 

 the rate are called heart inhibitors; and those that in- 

 crease the rate, heart accelerators. By their aid the rate 

 of flow is adjusted to the needs and conditions of the 

 tissue cells. 



Artery control. — The walls of the arteries, as well as 

 of the heart, are supplied with the ends of what are called 

 the vaso-motor nerves. Under the normal action of these 

 nerves the walls of the arteries have a certain elasticity 

 which responds to the blood pressure; stretching when 

 the pressure is increased and contracting as the pressure 

 is removed. When these vaso-motor nerves are stimu- 

 lated further the effect produced may be a still greater 

 contraction of the artery walls and a consequent decrease 

 in the blood content of the contracted artery, or a paralysis 

 of the walls which causes them to lose their elastic tension 

 and stretch. In this way it is possible for conditions, 

 external or internal, to affect these nerves and produce 

 stimuli, which, in turn, control the blood supply of any 

 given part of the body. The nerves which, when stimu- 

 lated, cause the contraction of the walls are called the 

 vaso-constrictors. Those which, when stimulated, pro- 

 duce paralysis of the walls and thus allow them to stretch 

 are called the vaso-dilators. This stretching and contract- 



