SUPERCOOLING AND FREEZING-ANDJUS, LOVELOCK axd SMITH 139 



The ranae of active defence. Two changes occur in animals maintained at 

 23-25° C. 



(1)1 )ni-ing the Hrst i)hase ( lasting ahout 20 hours ) , the animal actively attempts 

 to rewarm. It is capable of regaining spontanecjusly its normal body temperature if 

 placed in air and at a temperature somewhat lower than its body temperature. The 

 longer, however, the period of continuous hypothermia prior to the beginning of 

 rewarming, the longer it takes for si)ontaneous recovery (five and eight hours if 

 rewarming is initiated after 8 and 20 hours of hypothermia, respectively). 



(2) During the second phase (20-30 additional hours), the animal is deprived 

 of the capability of spontaneous rewarming. If rewarmed artificially by external 

 heat during the first few hours of this poikilothermic phase, the animal recovers 

 completely. Later still, it does not respond to passive external rewarming. It dies 

 during such rewarming, usually before normal body temperature is reached. In 

 other words, long before the rat dies of continuous hypothermia, the possibility of 

 recovery by simple rewarming is lost. 



Figure 7 illustrates the above phenomena in terms of body temperature. Unanes- 

 thetized rats were maintained in hypothermia by immersion up to the neck in water 

 of the same temperature as the cooled body. During the first 21 hours during 

 which the animal is capable of spontaneous rewarming in air, the body temperature 

 levels out at about two degrees above the temperature of the surrounding water 

 after an initial four hours' period of maximal rise, corresponding to a peak of oxy- 

 gen consumption. The second phase, during which spontaneous rewarming in air is 

 impossible, is initiated by a narrowing of the temperature difference to about one- 

 half a degree Centigrade. Figure 8 shows that during the first five hours of immer- 



o 



o 



15 20 25 30 35 40 



HOURS 



Fig. 7.— Changes in course of maintained hypothermia. Rats cooled by the closed vessel technique 

 to a given body temperature and then immersed in water of the same temperature. 



