288 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



result of this self-sacrifice we are in a position to evaluate Allen's 

 rule in man. 



Being a warm-blooded animal is a great advantage. It permits one 

 to move and act at nearly all times in nearly all places, instead of 

 scampering feverishly for shade or waiting for the chill to burn off 

 before moving. However, the process of keeping the internal organs 

 at a temperature of 98.6° F. has its problems too. This temperature 

 can fall to 77° F. or rise to 110° F. before death intervenes, but varia- 

 tions of half these magnitudes are serious, particularly on the high side, 

 for man can lose heat more safely that he can gain it. Even when he is 

 trying to keep warm, man loses a certain amount of heat functionally 

 in evaporation of moisture through the palms, soles, axillae, and pubic 

 regions, just to keep tactile and hinge areas ready for action. 



As long as the temperature of the outside environment is below 

 83° F., the body normally loses heat by radiation and convection. 

 At 83° F. it begins to sweat, and the surface of the body grows increas- 

 ingly moist, until at 93° F., in a saturated atmosphere, the whole body 

 is covered, water is dripping off the surface, and the perspiration 

 fails to do its work, which is to cool the surface of the skin by evapora- 

 tion. At this point, if the temperature rises without a drop in 

 humidity, trouble is near. However, in dry air only 40 percent of the 

 body surface is normally wet at 93° F. ; at blood temperature the ratio 

 is 50 percent, and a complete coverage, in the American human guinea 

 pig, is not attained until 106° F.^ 



The evaporation of sweat is the principal means by which the body 

 loses its radiant heat. Experiments have shown that a resting man at 

 122° F. and a humidity of 44 percent will lose 1,798 grams of sweat 

 per hour; a working man, in a humidity of 35.6 percent saturation, 

 will lose 3,880 grams per half hour, or half his normal blood volume, 

 at a cooling potential of 25 to 30 times the normal resting metabolism. 

 Needless to say such a liquid turnover requires him to drink gallons of 

 water and also taxes his heart. It is greatly to the advantage of human 

 beings living under conditions of extreme heat to avoid this circum- 

 stance as much as possible. 



Such heat is found largely in the deserts of the world,® which lie 

 on either side of the Equator, on the Tropics of Cancer and Capricorn. 

 Chief among them are the Sahara, the Arabian, Persian, Thar, Kala- 

 hari, Australian, Argentine, Chilean, and Colorado Deserts. Of these 

 the Turkestan, Gobi, Argentine, and Colorado Deserts lie farthest 

 from the Equator. Characteristic of deserts is a great diurnal varia- 

 tion in temperature, and often a seasonal one as well. On a hot day 



•Baiett, 1949; Best and Taylor, 1948 ; Day, 194&; Hardy, 1949; Herrington, 1949 ; 

 Robinson, 1949; Spealman, 1949a. 



• Adolph, 1947 ; Brooks, 1930 ; Buxton, 1923. 



