REGULATING THE HEAT 155 



which serve to regulate combustion in the body of which 

 we have no knowledge as yet. 



There remains to be mentioned a very important 

 mechanism for controlling the heat of the body. All 

 over the skin are placed minute transmitting stations — or 

 temperature transmitters. Some are sensitive to those 

 conditions we regard as cold, others are sensitive to con- 

 ditions which give us the feeling of warmth. Messages 

 are being continually taken up at these receiving stations 

 concerning the state of the surrounding temperature and 

 transmitted to central exchanges in the spinal cord and 

 brain stem. From these central exchange stations other 

 messages are dispatched to all parts of the body regulating 

 and controlling the rate at which their combustion is 

 carried on. A cold day sets messages on foot that speed 

 up combustion ; the messages of a warm day damp down 

 the sources of heat. The skin, then, is the end organ of 

 a great heat-regulating mechanism. How we stand heat 

 and cold depends on how perfectly this mechanism reacts. 

 Man is the most naked of all living hot-blooded machines. 

 There was a time, before he learned to clothe his body, 

 when his comfort — his power to live — depended on the 

 sensitiveness and effectiveness of this skin-regulating 

 mechanism. It was a costly contrivance, because it con- 

 sumed so much of the fuel of the body. In modern days 

 we depend on it less because we have learned to make 

 warm clothes and build houses in which a more equable 

 temperature can be maintained. 



So far we have been speaking of the mechanism for 

 controlling heat-production in the body. We come now 

 to an equally important matter — the contrivances which 

 Nature has adopted to regulate the escape of heat so that 

 the mass of the body will be maintained at a temperature 

 between 9 8° and 99 F. Her chief contrivances to secure 

 this end have been worked out in the skin. The human 

 skin has been made into leather and used for binding 

 books ; the tanner rubs off the friable surface layer or 

 epidermis ; the deeper stratum or cutis, a tough fibrous- 

 textured material, forms the leather. The skin of a man 



