426 DEVELOPMENT OF HEAT, ETC. 



temperature of the surrounding medium. As a general rule, the 

 Invertebrated animals are cold-blooded; that is, they have little or no 

 power of sustaining an independent temperature. -The degree of 

 energy of their vital actions entirely depends, therefore, upon the 

 warmth they receive from the air or water they inhabit ; they have no 

 power of resisting the depressing influence of cold ; and they are gene- 

 rally so organized, as to pass into a state of complete inaction or torpi- 

 dity, when the temperature sinks below a certain point, after gradu- 

 ally becoming more arid more inert with every diminution in the heat 

 of their bodies. The same is true, also, of most Fishes and Reptiles : 

 but the animals of the former class, from the more equable temperature 

 of the medium they inhabit, are not so liable to be reduced to inaction 

 as the latter ; being usually so organized, as to retain their activity so 

 long as the water around them continues liquid; and being actually 

 imbedded in a frozen state, when the water around them is converted 

 into ice, without the loss of their vitality. There are certain Fishes, 

 however, such as the Thunny, Sword-fish, and other large species of 

 the Mackerel tribe, which are able to sustain a temperature consi- 

 derably above that of the sea they inhabit ; thus in the Bonito, the 

 heat of the body has been found to be 99, when the temperature of 

 the surrounding sea was but 80J. It is not probable, however, that 

 the temperature of the body would be kept up to the same standard, 

 if that of the sea should be considerably lowered ; but it would proba- 

 bly remain at from 18 to 20 above the latter. And in like manner, 

 it has been noticed that many of the more active Reptiles possess the 

 power of sustaining the temperature of their bodies at 10 or 15 above 

 that of the surrounding air. 



761. .The classes of animals which are especially endowed with the 

 power of producing and maintaining heat, are Insects, Birds, and 

 Mammalia. The remarkable variations which present themselves in 

 the temperature of the first of these classes, and the connexion of these 

 variations with the condition of the animals in regard to activity or 

 repose, have already been sufficiently noticed ( 123). The tempera- 

 ture of Birds is higher than that of any other class of animals ; varying 

 from 100 to 111 or 112. The lowest degree is found in some of the 

 aquatic species, as the Gull, and in those which principally live on the 

 ground, as the Fowl tribe ; and the highest in the birds of most active 

 flight, as the Swallow. The temperature of the Mammalia seems to 

 range from about 96 to 104 ; that of Man has been observed as low 

 as 96|, and as high as 102. The variations are dependent in part 

 upon the temperature of the external air ; but are influenced also by 

 the general condition of the body as to repose or activity, the period of 

 the day, the time that has elapsed since a meal, &c. A somewhat 

 larger amount of caloric is generated during the day, than in the night ; 

 and the body is usually warmer, by a degree or two, at noon, than at 

 midnight. There is also a slight increase during the digestion of a 

 meal ; and exercise is a powerful means of raising the temperature. 

 The range of temperature is much greater in disease ; thus the thermo- 

 meter has been seen to rise to 106 in Scarlatina and Typhus, and to 



