THE VASOMOTOR MECHANISM 339 



flexly. The whole stream of afferent impulses passing through 

 the medulla plays upon it. Like the centers for controlling the 

 heart-beat its activity may be increased through the influx of 

 stimuli into it, or it may suffer depression for the same cause. 

 We divide afferent impulses affecting the vasoconstrictor center, 

 therefore, into two groups: those increasing its activity, pressor 

 impulses, and those diminishing it, depressor impulses. Certain 

 sorts of stimuli are uniformly pressor in effect; pain, for example, 

 always brings about a reflex rise of blood-pressure through stim- 

 ulating the vasoconstrictor center. It is possible that other stimuli 

 may be pressor or depressor according to circumstances. 



The Depressor Nerve. The best-known nerve-tract which car- 

 ries depressor impulses uniformly has already been mentioned. 

 It is the afferent tract from the heart known, in animals where it 

 is present as a separate trunk, as the depressor nerve. Stimu- 

 lation of this nerve brings about, always, a reflex fall of blood- 

 pressure, which is due solely to vasodilatation resulting from de- 

 pression of the vasoconstrictor center. This nerve rises, not in 

 heart tissue proper, but in the walls of the aorta near where that 

 vessel springs from the heart. An undue increase in blood- 

 pressure, such as might affect the heart injurously, subjects 

 the aortic wall to unusual tension. This seems to stimulate 

 the depressor nerve mechanically. Thus the heart is protected 

 against injury arising from working against too great resist- 

 ance. 



Blushing is a vasodilatation involving chiefly the artcrioles of 

 the skin of the face and head. It, and similar local vasodilatations 

 of skin or visceral regions, are due to depression of one or more 

 of the subordinate centers, which together make up the vaso- 

 constrictor center as a whole. The result of such local vaso- 

 dilatation is to increase very much the blood supply of the region 

 involved, without much effect on the rest of the Body. Blushing 

 is one of the most familiar examples of an emotional reaction in- 

 volving the sympathetic system. A case of an adaptive vaso- 

 dilatation is the reddening of the hand when placed in hot water. 

 In this reflex the sensory stimulus is the heat applied to the skin. 

 It depresses that part of the vasoconstrictor center which controls 

 the arterioles of the hand and their relaxation follows. The result 

 of the increased blood-supply to the hand is a more rapid loss of 



