376 THE HUMAN BODY 



part reflexly, and in part, probably, through chemical stimulation 

 brought by way of the blood. The whole stream of afferent im- 

 pulses 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 vasocon- 

 strictor center, therefore, into two groups: those increasing its 

 activity, pressor impulses, and those diminishing it, depressor 

 impulses. Certain sorts of stimuli are generally pressor in effect; 

 pain, for example, usually brings about a reflex rise of blood- 

 pressure through stimulating the vasoconstrictor center; cold on 

 the skin acts similarly. It is possible that other stimuli may be 

 pressor or depressor according to circumstances. 



The Depressor Nerve. The best known nerve-tract which 

 carries 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. Stimulation 

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

 which is due mainly to vasodilation resulting from depression 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 injuriously, subjects the aortic wall to un- 

 usual tension. This seems to stimulate the depressor nerve me- 

 chanically. Thus the heart is protected against injury arising 

 from working against too great resistance. 



Taking Cold. This common condition is not unfrequently the 

 indirect result of undue reflex excitement of the vasomotor center. 

 Chilling of the skin beyond a certain point stimulates, through the 

 afferent nerves, the portion of the vasomotor center governing the 

 skin arteries, and the latter become contracted, as shown by the 

 pallor of the surface. This has a twofold influence in the first 

 place, more blood is thrown into internal parts, and in the second, 

 contraction of the arteries over so much of the Body considerably 

 raises the general blood-pressure. Consequently the vessels of 

 internal parts become overgorged or "congested," a condition 

 which is especially favorable to invasion by the organisms which 

 cause colds. The best preventive is to wear, when exposed to 

 great changes of temperature, a woolen or at least a cotton gar- 



