156 THE ENGINES OF THE HUMAN BODY 



of medium size, when removed and spread out, would 

 make a sheet a little over 4 feet square in extent — 

 about 17 square feet. In life these 17 feet of cutis form 

 a vascular sheet, a great capillary field, flooded with a 

 rich supply of blood. The arterioles supplying the field 

 with blood are provided with the most sensitive of stop- 

 cock mechanisms. These mechanisms are controlled by 

 centres set in the spinal cord and brain. The controlling 

 centres are supplied with messages from the temperature 

 transmitters which, as we have seen, are set everywhere in 

 the skin. On a warm day the stopcocks are opened, the 

 skin is flooded with blood ; heat radiates from the surface 

 of the body ; currents of air rise from the suffused skin 

 and rob the body of its heat. On a cold winter day the 

 opposite happens ; the stopcocks are turned off; the 

 skin becomes pale and bloodless ; the radiation of heat 

 is diminished. Physiologists estimate that about three- 

 fourths of the heat produced in the body escapes by way 

 of the skin. 



What happens if we are placed in surroundings which 

 are warmer than the normal temperature of the body ? 

 A man may live in a chamber which is kept hot enough to 

 cook a fowl and yet his temperature may rise only a degree 

 or two above the normal. If his body were not a living 

 thing it would absorb heat and become cooked. Nature 

 has used the simple contrivance of evaporation to keep the 

 body cool. A drop of sweat evaporated on the skin takes 

 away from the body as much heat as would be sufficient 

 to raise two other drops above the boiling-point of water. 

 Sweating robs the body of heat and thus cools it. Hence 

 the surface of the skin is studded with the openings of 

 sweat glands ; in most areas of the body there are about 

 500 sweat glands to a square inch of skin ; in the sole 

 of the foot and palm of the hand they are four or five 

 times more numerous. The sweat glands, like the blood- 

 vessels of the skin, are controlled from nerve mechanism 

 in the spinal cord and brain stem. If the blood supplying 

 these control centres becomes warmer than the normal 

 body temperature, their nerve cells are stimulated and 



