84 EXTERNAL CONDITIONS OF VITAL ACTIVITY. 



place within the body in autumn, favoured by the oily nature of the 

 seeds, nuts, &c., on which the animals then feed ; and this serves the 

 purpose of maintaining the temperature for a sufficient length of time, 

 not indeed to the usual standard, but to one not far below it. The 

 state of torpor in these animals is more profound than that of deep sleep, 

 but it is not such as to prevent them from being easily aroused ; and 

 their respiratory movements, though diminished in frequency, are still 

 performed without interruption. But in the Marmot, and in animals 

 which, like it, hybernate completely, the temperature of the body (owing 

 to the want of internal power to generate heat) and the general vital 

 activity, are proportionally depressed ; the respiratory movements fall 

 from 500 to 14 per hour, and are performed without any considerable 

 enlargement of chest ; the pulse sinks from 150 to 15 beats per minute ; 

 the state of torpidity is so profound, that the animal is with difficulty 

 aroused from it ; and the heat of the body is almost entirely dependent 

 upon the temperature of the surrounding air, not being usually more 

 than a degree or two above it. When the thermometer in the air is 

 somewhat below the freezing-point, that placed within the body falls to 

 about 35; and at this point it may remain for some time, without any 

 apparent injury to the animal, which revives when subjected to a higher 

 temperature. When, however, the body is exposed to a more intense 

 degree of cold, the animal functions undergo a temporary renewal; for 

 the cold seems to act like any other stimulus in arousing them. The 

 respiratory movements and the circulation increase in activity, so as to 

 generate an increased amount of heat ; but this amount is insufficient to 

 keep up the temperature of the body, which is at last depressed to a 

 degree inconsistent with the maintenance of life ; and not only the sus- 

 pension of activity, but the total loss of vital properties, is the result. 



122. Now the condition of a hybernating Mammal closely resembles 

 that of a cold-blooded animal, in regard to the dependence of its bodily 

 temperature upon external conditions. There is this important diffe- 

 rence, however ; that the reduction of the temperature of the former 

 to 60 or 50 is incompatible with a state of activity, which is only exhi- 

 bited when the temperature rises to nearly the usual Mammalian standard ; 

 whilst a permanently low or moderate temperature is natural to the 

 bodies of most cold-blooded animals, whose functions could not be well 

 carried on under a higher temperature. Thus all the muscles of a Frog 

 are thrown into a state of permanent and rigid contraction, by the 

 immersion of its body in water no warmer than the blood which naturally 

 bathes those of the Bird ; and we find, accordingly, that cold-blooded 

 animals which cannot sustain a high temperature, are provided with a 

 frigorifying rather than with a calorifying apparatus. Although we 

 are accustomed to rank all animals, save Birds and Mammals, under the 

 general term cold-blooded, yet there exist among them considerable diver- 

 sities as to the power of generating heat within themselves, and of thus 

 rendering themselves independent of external variations. Thus among 

 Reptiles, it appears that there are some which can sustain a temperature 

 several degrees above that of the atmosphere, especially when the latter 

 is sinking ; and among Fishes, it is certain that there are species, the 

 Thunny and Bonito for example, which are almost entitled to the 



