202 NUTRITIONAL PHYSIOLOGY 



that high humidity in hot weather interferes with our com- 

 fort and efficiency by hindering free evaporation. In 

 winter it has no such influence, because even though the 

 cold air is fully saturated it ceases to be so when it has been 

 warmed by contact with the skin. Evaporation can, 

 therefore, never be retarded seriously by moisture in cool 

 air. What we notice in cold weather is the increased con- 

 ducting power of air containing water vapor. Damp air 

 may fairly be said to partake of the nature of the water 

 that is in it; water feels colder to the hand than does air of 

 the same temperature because it abstracts the heat more 

 rapidly. Similarly, moist cold air takes heat more rap- 

 idly than does dry cold air. This property is present 

 just as surely in warm humid air, but it can affect us only 

 when there is a wide difference in temperature between the 

 skin and the surroundings. 



Temperature Maintenance During Exercise. We have 

 been discussing the ways in which the human body guards 

 itself against changes of temperature which tend to impress 

 themselves upon it from the outside. Another question 

 is in regard to how it escapes the tendency to overheat 

 itself when, during exercise, its metabolism is doubled, 

 trebled, or even more strikingly augmented. Reflection 

 shows that it employs the two reflexes on which it relies 

 for defense against the heat of the warm room, namely, 

 increased surface blood-flow and increased perspiration. 

 These are rendered more efficient by the hastened circula- 

 tion, a condition not produced in any great degree by ex- 

 ternal heat without the activity. 



A supplementary factor exists in the deepened breathing 

 which takes heat from the system, both in the act of warm- 

 ing the respired air and in the process of saturating it. 

 Still another factor can be recognized in the fanning effect 

 of the movements of parts of the body or of the body as a 

 whole. A man running brings the exposed portions of his 

 skin constantly into contact with fresh volumes of air and 

 slips away from the air which he has just warmed and 

 saturated. The cooling of his blood is in this way con- 

 siderably facilitated. If he is not progressing through 



