162 THE HUMAN MECHANISM 



Two means are known by which the contraction of the 

 circular muscle fibers of the arterioles is regulated: first, 

 impulses from the central nervous system over the vasomotor 

 nerves; and, second, direct excitation of the arterioles by 

 hormones in the circulating blood. The vasomotor nerves 

 are of two kinds, the vasoconstrictors and the vasodilators; 

 the best-known hormone acting on the arterioles is adre- 

 naline, the action of which has already been referred to in 

 Chapters VI and VII. 



26. Vasoconstrictor nerves. The muscle fibers of the arter- 

 ies receive nerves which stimulate them to contract, for if 

 these nerves are cut, the arteries lose their tone (dilate). 

 We conclude, therefore, that the ordinary maintenance of 

 arterial tone is, in part at least, a function of the nervous 

 system. The muscle fibers of the arteries, in other words, 

 remain in tonic activity because the neurones which supply 

 them with nerve fibers are in tonic activity; and we can 

 understand how general arterial tone may be increased or 

 decreased by the condition of the central nervous system, by 

 reflexes, by the nervous " shock " of surgical operations, etc. 



Neurones which maintain the proper amount of arterial 

 tone are known as vasoconstrictor neurones. They obviously do 

 for the muscles of the arteries what the motor nerves do for 

 the skeletal muscles, and the augmentors do for the heart. 



27. Vasodilator nerves. Many arteries, however, receive a 

 second set of nerves, which have exactly the opposite func- 

 tion, that is, to make their muscle fibers relax and so lead to 

 a widening or dilation of the artery. These nerves do for the 

 tonic contraction of the arteries what the inhibitory nerves of 

 the heart do for the heart beat they diminish or abolish 

 an existing activity and thus give us our second example 

 of inhibitory nerves. They are known as the vasodilators. 



The vasodilators are not regularly in tonic activity like 

 the vasoconstrictors. They are called into action, reflexly or 

 otherwise, when it is necessary that an organ receive more 



