THE AUTONOMIC NERVOUS SYSTEM 123 



contraction has occurred in the microscopic veins, holding 

 back the blood in the capillaries, which accordingly swell 

 even though the arteries leading to them may be below 

 their average size at the time. (It is the amount of blood 

 in the capillaries which determines the color of the surface.) 



The capacity of the vasomotor system to make appro- 

 priate responses to changes of external temperature is far 

 from equal in different subjects. What we call "vital 

 resistance"- the power to withstand exposure without 

 ill effects must depend largely upon the extent of this 

 capacity. It is probable that the variable feature is not 

 so much in the execution of the primary reaction to weather 

 changes, as in the reliability of the secondary adjustment, 

 the promptness and vigor of the rebound to normal vaso- 

 motor conditions after a disturbance. The person who 

 quickly develops a "glow" after a cold bath is a living 

 demonstration of such a vasomotor resilience, and we know 

 that such a person is not likely to take cold or to be other- 

 wise upset after being caught in a shower or obliged to sit 

 in a chilly place. Still we must not fail to reckon with other 

 factors than the vasomotor when we compare susceptibili- 

 ties; there are always chemical as well as mechanical differ- 

 ences between two human bodies, and the bacteriologist 

 refers to peculiarities of chemical endowment when he 

 speaks of the superior resistance of this or that man to 

 infecting agents. It is well for us to keep both considera- 

 tions in sight. If two men fall through the ice and both 

 escape unfortunate consequences, it is entirely con- 

 ceivable that the immunity in one case may be due to 

 vasomotor reactions which guard against the persistence 

 of conditions which invite infection, and in the other to 

 specific properties of the blood. 



Something should be said of the vasomotor adjustment 

 for changes in position. This is a matter often ignored 

 by the writers of physiologic text-books, but it does not 

 require much reflection to convince one that it is of high 

 importance. The inevitable influence of gravity on the 

 blood is to add to the pressure in all vessels below the 



