254 ANIMAL HEAT. 



heat takes place with such activity that their blood and internal 

 organs are nearly always very much above the external temper- 

 ature; and which are therefore called "warm-blooded animals." 

 These are mammalia and birds. Among the birds, some species, 

 as the gull, have a temperature as low as 100 F. ; but in most of 

 them, it is higher, sometimes reaching as high as 110 or 111. In 

 the mammalians, to which -class man belongs, the animal tempera- 

 ture is never far from 100. In the seal and the Greenland whale, 

 it has been found to be 104 ; and in the porpoise, which is an air- 

 breathing animal, 99.5. In the human subject it is 98 to 100. 

 When the temperature of the air is below this, the external parts 

 of the body, being most exposed to the cooling influences of radia- 

 tion and conduction, fall a little below the standard, and may indi- 

 cate a temperature of 97, or even several degrees below this point. 

 Thus, on a very cold day, the thinner and more exposed parts, such 

 as the nose, the ears, and the ends of the fingers, may become 

 cooled down considerably below the standard temperature, and may 

 even be congealed, if the cold be severe ; bat the temperature of 

 the internal organs and of the blood still remains the same under 

 all ordinary exposures. 



If the cold be so intense and long continued as to affect the 

 general temperature of the blood, it at once becomes fatal. It has 

 been found that although a warm-blooded animal usually preserves 

 its natural temperature when exposed to external cold, yet if the 

 actual temperature of the blood become reduced by any means 

 more than 5 or 6 below its natural standard, death inevitably 

 results. The animal, under these circumstances, gradually becomes 

 torpid and insensible, and all the vital operations finally cease. 

 Birds, accordingly, whose natural temperature is about 110, die if 

 the blood be cooled down to 100, which is the natural temperature 

 of the mammalia ; and the mammalians die if their blood be cooled 

 down below 94 or 95. Each of these different classes has there- 

 fore a natural temperature, at which the blood must be maintained 

 in order to sustain life ; and even the different species of animals, 

 belonging to the same class, have each a specific temperature which 

 is characteristic of them, and which cannot be raised or lowered, to 

 any considerable extent, without producing death. 



While in the birds and mammalians, however, the internal pro- 

 duction of heat is so active, that their temperature is nearly always 

 considerably above that of the surrounding media, and suffers but 

 little variation ; in reptiles and fish, on the other hand, its produc- 



