=Animal Heat.=--The bird is warmer than the air in which it moves; the fish possesses a higher temperature than the water. As before remarked, chemical changes are constantly going on in the system which give rise to this result. Even plant life is subject to this law. A delicate thermometer placed among a cluster of geraniums about to burst into flower will show a temperature a degree or two higher than the surrounding air.
=Warm-Blooded Animals.=--Those animals possessing well-developed lungs and large breathing capacity are usually active in movement, and are classed among the warm-blooded animals. They comprise birds, quadrupeds, and man. The animals possessing small lung development are for the most part inactive, and are cold to the touch, indicating a low temperature. Such are the frog, toad, lizard, snake, and tortoise.
These facts show the connection between respiration and animal heat, the temperature being in proportion to the amount of oxygen consumed. Birds have the largest lung development in proportion to size, are most active in movement, and indicate the largest amount of animal heat.
=Adaptation to Climatic Conditions.=--The Polar bear suffers from the heat of the Temperate zone, and would not survive a week in the Torrid. The African lion would fare no better were he suddenly transported to the Frigid zone. Man alone, of all the animal creation, is able to adapt himself to the extremes of heat and cold. By changing his clothing, shelter, and food, he is able to create for himself an artificial climate wherever he may choose to reside. No Arctic winter has been found too cold for a Peary, Nansen, or Greely to withstand, and no African plain or jungle too hot for a Livingstone or Stanley to explore.
=Evaporation.=--The temperature of the body is regulated by means of perspiration. Heat induces perspiration, and its evaporation lowers the temperature of the body. Cold retards perspiration, and the heat is retained within. The principle of evaporation is illustrated in the manufacture of artificial ice. Men who labor in glass works, iron and steel foundries, and in the engine rooms of large steam vessels are exposed to great heat, yet enjoy as good health as those who are engaged in other occupations. Persons have been known to remain several minutes in an atmosphere heated above the boiling point, without materially increasing the temperature of their own bodies.