290 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1953 



Man in the desert is also light and gracile. He too needs to be able 

 to travel far on a small heat load. But his animal companions have 

 buff -colored hairy coats, which reflect solar light ; it is unlikely that 

 they lose their heat in this fashion. Man must lose it through his 

 skin surface, and the more surface he has per unit of weight the better. 

 The more he can lose through radiation and convection the less he 

 has to sweat, and the more skin surface he can use for evaporation the 

 higher the temperature he can stand. The smaller his bulk, the less 

 the load on his heart. The shape of his body takes on added import- 

 ance as we realize that all parts of its surface do not lose heat equally. 

 The back of his hand has about 400 sweat glands to the square centi- 

 meter, the forehead 200, and the cheek as few as 50. The hands, 

 which comprise 5 percent of the body surface on normal Americans, 

 lose 20 percent of the heat of the body by evaporation (Bazett, 1949, 

 p. 131). 



"VAHien a man begins to perspire, moisture appears first on his fore- 

 head, neck, some of the larger areas on the front and back of his 

 trunk, the back of the hand, and the adjacent part of the forearm. 

 The head and neck must lose heat rapidly for they have the brain to 

 keep in thermal equilibrium, and if the head is globular in form, it 

 has the worst possible shape for heat loss. Old World hot-desert 

 peoples are narrow lieaded. After this the cheek, the lateral surfaces 

 of the trunk, and the rest of the extremity surfaces begin, but these 

 regions sweat much less. Sweating is always slight to moderate on 

 areas rich in subcutaneous fat, such as the cheek and the gluteal and 

 mammary regions. The inside of the thighs and armpits sweat even 

 less, since they face in and not out and are in a poor position for heat 

 loss. The palms and soles, which perspire at lower temperatures, lose 

 the least of all in periods of stress. 



The chief burdens then are on the neck and head, which haA^e purely 

 local duties, and on the hands and forearms, which act as radiators 

 for the whole body. It has been shown that the average human 

 body (American) loses heat after the fashion of a cylinder averaging 

 7 cm. in diameter (Hardy, 1949, p. 97) . While the head and trunk are 

 bulkier than this, the forearms and hands resemble even smaller cylin- 

 ders, and the fingers and toes even smaller yet. Now heat loss increases 

 as the square of the diameter of the cylinder decreases. Hence the 

 survival value of long, tapering forearms and fingers in a dry, hot 

 place becomes self-evident. 



One of the racial pecularities of Negroes is long arms, with par- 

 ticular emphasis on the length of the forearm, and large hands with 

 long fingers. Forest Negroes often have relatively short legs, but 

 we have seen that the legs have much less to do with heat regulation 



