THE REGULATION OF THE CIRCULATION 269 



fluence of the vasodilator fibers are of a paralyzing sort, 

 and the vessel is left free to become stretched by the 

 stream that is being driven through it. 



It will now be appreciated that the blood-vessels, like 

 the heart, may be played upon by nerve-impulses having 

 opposite effects. The vagus fibers inhibit the heart; 

 the dilator fibers inhibit the blood-vessels. The heart 

 has accelerator fibers which reinforce its contractile 

 activity and the tone of the small vessels can be rein- 

 forced through the constrictor fibers. It should be 

 added that in the case of the heart the inhibitory in- 

 fluence is more in evidence than the opposing one, while 

 the constrictor effect is more prominent than the dilator 

 among the small vessels. Vasodilators do not seem to 

 have a universal distribution. Their existence is best 

 established for the glands, the muscles, the skin of the 

 face, and the genitals. 



Vasomotor Adjustments. We are now in a position 

 to consider some of the adaptive changes made by the 

 nervous mechanisms presiding over the circulation. 

 The great purpose to be subserved is to provide an ample 

 supply of blood to the various capillary areas and to 

 meet the varying demands of different regions. On 

 the negative side we may recognize that economy is 

 secured by restricting the flow through parts where 

 there is no pressing need. The general principle is to 

 increase the supply to any seat of activity, motor or 

 secretory, and to limit the amount of blood sent through 

 tissues which are temporarily at rest. 



A local demand, such as may be made by the salivary 

 glands during mastication, may be satisfied by a lower- 

 ing of the tone in the vessels of these organs and may 

 not entail an appeal to the heart. A widespread need, 

 like that of muscular exercise, requires not only the 

 dilation of many vessels, but a general speeding up of 

 the circulation which can only be maintained by in- 

 creased heart action. (When vigorous exercise is taken 

 the dilation must extend to the skin as well as the muscles. 



