322 ELEMENTS OF PHYSIOLOGY 



activity of the chick, the beginning of the habit of breath- 

 ing, and the drying of its downy feathers. 



583. The Loss of Heat occurs by conduction through the 

 skin, also by the evaporation of water from the skin and 

 lungs, and by the taking in of cool food, drink, and air. 



584. How the Nerves regulate the Heat. The tempera- 

 ture of the body is raised by increasing the heat produc- 

 tion, or by diminishing the loss of heat. The temperature 

 is lowered by the opposite processes. There are believed 

 to be three classes of nerve centers to regulate the pro- 

 cesses. One class of centers in the brain when stimulated 

 by cold causes the heat production to increase ; another 

 class when stimulated, causes heat production to diminish. 

 The third center is in the spinal cord and is automatic, but 

 may be influenced by the centers in the brain. We should 

 be in danger of being killed by the cold long before the 

 thermometer reached the freezing-point, if it were not true 

 that cold, in whatever form it may be applied, starts im- 

 pulses in the skin which excite the tissues that generate 

 heat, so that there is increased production of heat. 



585. The Distributor of Heat generated by the active 

 organs, to the resting organs, is the blood. The blood 

 also causes heat to leave the body, giving it off while pass- 

 ing through the blood vessels of the skin. 



586. How does perspiration cool the body ? Why is it 

 an advantage for you to shiver and for your teeth to 

 chatter when you are very cold ? Why is it an advantage 

 for the skin to turn pale when one goes out into a cold 

 wind ? What effect upon perspiration does the blanching 

 of the skin have ? What effect does the blanching have 

 upon the loss of the heat of the body from the contact 

 with the cold air ? 



587. Clothing conserves the heat of the body, but cannot 

 generate it. Such heat arises from the combustion in the 

 system of the food by the oxygen of the air. In hot 



