ANIMAL, HEAT. 175 



both. Nor is our heat derived from clothing, foi this has no 

 active power of giving heat it has no warmth in itself; it 

 only tends to prevent changes of temperature. If we wrap a 

 piece of dead flesh in flannel, it is not warmed ; it remains 

 the same as before. If, in the winter, this flesh be heated 

 by fire, the flannel wrapped about it keeps it warm. If, in 

 summer, we put ice in flannel, it prevents the melting. 

 Clothing, then, only prevents the passage of heat. It keeps 

 a warm body warm, and a cool body cool; but it creates 

 and gives no heat. If, then, animal heat is not given from 

 without, it must originate within the body. There must be 

 some internal means or apparatus by which we and other 

 living beings create and sustain our temperature. 



410. There are two grand classes of animals, divided ac- 

 cording to their temperature. One is called the warm- 

 blooded, and includes man, birds, quadrupeds, &-c. Their 

 heat is ever of the same degree, and does not vary with the 

 temperature of the atmosphere or the water in which they 

 live. The other class is called cold-blooded, and includes 

 snakes, oysters, fishes, worms, toads, turtles, &/c. Their heat 

 is but little higher than that of the medium in which they live. 

 The earth-worm, leech, and shell-fish are usually 1J warmer 

 than the air, or earth, or water which surrounds them. 

 Fishes are 2 to 5 warmer than the water. Reptiles, frogs, 

 lizards, have a still higher heat relative to the air or water, 

 yet not so high and permanent as that of the warm-blooded 

 animals. 



411. There is, in these two great classes, a great differ- 

 ence of power of maintaining their own heat. Man main- 

 tains his usual temperature in the midst of air varying 320 

 from extreme heat to extreme cold ; and therefore he may be 

 at least 160 warmer, or 160 cooler, than the surrounding 

 medium ; while a fish is only 2 or 3 warmer or cooler than 

 the water in which it lives. It is natural, then, to ask, 

 What is the difference in the structure of these classes, from 

 which arises this difference of internal heat 1 On examina- 

 tion, we find that the principal difference that runs through 



