CHAPTER XXIV. 



ANIMAL HEAT. 



Part of the work done or energy set free by the chemical 

 changes in the animal tissues appears as heat which is devoted 

 to keeping the body warm ; for the bodies of most animals are 

 considerably warmer than their surroundings. Warm-blooded 

 animals are those which habitually preserve an even temperature 

 independent of the changes which take place in that of the 

 medium in which they live; and, as the term warm-blooded 

 implies, their temperature is, as a rule, higher than the sur- 

 rounding air or water. Cold-blooded animals, on the other hand, 

 are those whose temperature is considerably aifected by, or more 

 or less closely follows, that of the medium surrounding them. 



The blood of all mammalia has pretty much the same tempera- 

 ture as that of man, about 37.5 C., and probably varies under 

 similar circumstances. But birds, the other class of warm-blooded 

 animals, have a temperature about 4 or 6 C. higher than that of 

 mammals. 



The blood of those animals whose temperature follows the 

 changes that occur around them, is generally from 1 to 5 C. 

 higher than the medium in which they live. They produce some 

 heat, though it be in small quantity, and since they have no special 

 plan for its regulation, it does not remain at a fixed standard. 

 Everywhere that active oxidation takes place, heat is produced ; so 

 even in invertebrate animals an elevation of temperature occurs ; 

 this can be easily ascertained when they exist in masses, as bees, 

 an active hive sometimes reaching a temperature of 35 C. 



Instead of the term " warm-blooded," it is more accurate to 

 apply to animals whose temperature remains uniformly even, and 

 independent of their surroundings, the term " Homceothermic" (of 

 constant temperature), and to animals with temperatures varying 

 with their surroundings, "Poikilothermie" (of changing tempera- 

 ture), instead of the words warm- and cold-blooded. 



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