68 ELEMENTARY PHYSIOLOGY less. 



of the arteries with which it is connected are always 

 contracted and the vessels themselves constricted, the 

 sympathetic is more precisely characterised as a vaso- 

 constrictor nerve. Further, since merely cutting the 

 sympathetic leads to a dilation of the blood-vessels of the 

 ear, we are justified in assuming that vaso-constrictor 

 impulses are continually being sent oitt along this nerve, 

 whereby the arteries are kept continually in a condition of 

 slight or medium constriction. To this condition the 

 name is given of arterial " tone." Now this " tone " is 

 of great importance, for by its existence it at once becomes 

 possible to increase the blood-supply to any part of the 

 body as well as to diminish it. Did the arteries possess 

 no "tone " they would, under ordinary resting conditions, 

 be dilated to their full extent, and the part or organ they 

 supply with blood would be receiving a maximum supply 

 when at rest. But the organs of the body are never at 

 rest for long, and when tliey become active they require 

 an increased amount of blood which could not be supplied, 

 at least by a vaso-constrictor mechanism, but for the exist- 

 ence of this arterial tone. It would of course be possible 

 to increase the blood-supply by means of an increased 

 activity of the heart ; but this would aftect the supply to 

 every part of the body at the same time, and what is 

 really wanted is a localised variation in supply to meet tlie 

 varying needs of each part or organ. Thus the vaso- 

 constrictor nerves act by carrying more.of less of the same 

 kind of impulse, leading to increase or loss of tone and 

 hence lessened or increased l)lood-supply ; they do not 

 act, as is so frequently and erroneously imagined, by 

 carrying one set or kind of impulses to produce constric- 

 tion, and another set or kind to produce dilation. 



We have quoted blushing as being a characteristic and 

 familiar instance of the action of vaso-motor (vaso-con- 

 strictor) nerves. But other examples of exactly similar 

 action are met with througliout the whole body. Thus 

 when a muscle contracts, or when a salivary gland secretes 



