192 A N I M A L H E A T. 



the highest temperature, 111.25, being in the small species, 

 the linnets, &c. Among reptiles, Dr. John Davy found, that 

 while the medium they were in was 75, their average tem- 

 perature was 82.5. As a general rule, their temperature, 

 though it falls with that of the surrounding medium, is, in 

 temperate media, two or more degrees higher ; and though it 

 rises also with that of the medium, yet at very high degrees it 

 ceases to do so, and remains even lower than that of the medium. 

 Fish and Invertebrata present, as a general rule, the same 

 temperature as the medium in which they live, whether that be 

 high or low ; only among fish, the tunny tribe, with strong 

 hearts and red meat-like muscles, and more blood than the 

 average offish have, are generally 7 warmer than the water 

 around them. 



The difference, therefore, between what are commonly called 

 the warm- and the cold-blooded animals, is not one of abso- 

 lutely higher or lower temperature ; for the animals which to 

 us, in a temperate climate feel cold (being like the air or 

 water, colder than the surface of our bodies), would, in an ex- 

 ternal temperature of 100, have nearly the same temperature 

 and feel hot to us. The real difference is, as Mr. Hunter ex- 

 pressed it, that what we call warm-blooded animals (birds and 

 Mammalia), have a certain "permanent heat in all atmo- 

 spheres," while the temperature of the others, which we call 

 cold-blooded, is "variable with every atmosphere." 



The power of maintaining a uniform temperature, which 

 Mammalia and birds possess, is combined with the want of 

 power to endure such changes of temperature of their bodies 

 as are harmless to the other classes ; and when their power of 

 resisting change of temperature ceases, they suffer serious dis- 

 turbances or die. 



Sources and Mode of Production of Heat in the Body. 



In explaining the chemical changes effected in the process 

 of respiration (p. 180 ), it was stated that the oxygen of the 

 atmosphere taken into the blood is combined, in the course of 

 the circulation, with the carbon and the hydrogen of disin- 

 tegrated and absorbed tissues, and of certain* elements of food 

 which have not been converted into tissues. That such a com- 

 bination between the oxygen of the atmosphere and the carbon 

 and hydrogen in the blood, is continually taking place, is 

 made certain by the fact, that a larger amount of carbon and 

 hydrogen is constantly being added to the blood from the food 

 than is required for the ordinary purposes of nutrition, and 

 that a quantity of oxygen is also constantly being absorbed 



