l'2'2 THE NERVOUS SYSTKM AND ITS CONSERVATION 



"hyperemic." Congestion- are of two types, active and 

 passive. In both there i- a local excess of blood, but in the 

 active form it is blood swiftly flowing through widely 

 opened vessels, while in the passive sort it is blood arrested 

 or dammed up, a state which naturally suggests a venous 

 obstruction. 



The congestion produced in the skin by heat is active 

 in character. A glance at the swollen veins convinces 

 one that they are carrying a great volume of blood out of 

 the flushed region. The reaction is a serviceable one; it 

 tends to rid the body of heat, both by increasing radiation 

 and conduction from the skin and by sustaining the activ- 

 ity of the -\veat-glands, which at such a time secrete much 

 water to be evaporated. If the external temperature is so 

 high that the body is cooler than its surroundings, the 

 radiation and conduction transfers may be reversed, but 

 it remains important to have an abundant circulation in 

 support of the secretion of sweat. There is at present 

 some doubt a.s to how far the increase of blood-flow which 

 heat produces in the skin is a reflex and how far a direct 

 effect of the high temperature. 



\\heu the skin is moderately cooled, it becomes pale, 

 the obvious sign of diminished blood-flow. This prevents 

 a wasteful loss of heat to the exterior and so conserves the 

 bodily resources. But everyone knows that more pro- 

 nounced cooling will shortly induce a reddening. How 

 does the redness produced by cold differ from that pro- 

 duced by heat? Investigation of this question is easily 



c hided. If one hand is put into very cold water and 



the other into water as hot as can be borne, both hands 

 will lie red, but certain differences will be quickly apparent . 

 It has been said that the veins in the heated hand are 

 swollen; in the chilled hand they are small and dark. This 

 fact, taken in connection with the tendency of the cold 

 skin to become bluish, shows that the congestion produced 

 by cold is of ;i passive nature. Much blood is in the area, 

 but it is making only slow progress through it. The con- 

 dition i> best accounted for on the theorv that a marked 



