ANIMAL HEAT $77 



graduation of the apparatus. One advantage of an air calorimeter 

 is that it follows more closely rapid variations in the heat-production 

 of the animal, or, to speak more correctly, in the heat loss. It 

 should be carefully noted that in calorimetry what is directly 

 measured is the quantity of heat given out by the animal, not the 

 quantity produced. The two quantities are identical only when the 

 temperature of the animal has remained unchanged throughout the 

 experiment. If the temperature has fallen, the quantity of heat 

 produced is equal to the quantity measured by the calorimeter minus 

 the difference between the quantity in the animal at the beginning 

 and at the end of the observation. This difference is equal to the 

 average specific heat of the animal multiplied by its weight and by 

 the fall of temperature. It can be approximately found by multiply- 

 ing the weight (in kilogrammes or grammes) by the fall of rectal 

 temperature (in degrees), since the average specific heat of the body 

 is not very different from that of water, and the specific heat of water 

 is taken as unity. 



All the higher animals (mammals and birds) have a prac- 

 tically constant internal temperature (fowl 41 to 44 C., mouse 

 37 to 38, dog 38 to 39, man 37 in the rectum), but a few 

 hibernating mammals, such as the marmot, are homoiothermal 

 in summer, poikilothermal during their winter sleep. In the 

 lower forms the body-temperature follows closely the tempera- 

 ture of the environment, and is never very much above it (frog 

 0-5 to 2 above external temperature). Both in a frog and in 

 a pigeon heat is evolved as long as life lasts ; but per unit of 

 weight the amphibian produces far less than the bird, and loses 

 far more readily what it does produce. The temperature of 

 the frog may be 30 C. in June and 5 in January. The structure 

 of its tissues is unaltered and their vitality unimpaired by such 

 violent fluctuations. But it is necessary, not only for health, 

 but even for life, that the internal temperature (the tempera- 

 ture of the blood) of a man should vary only within relatively 

 narrow limits around the mean of 37 to 38 C. 



Why it is that a comparatively high temperature should be 

 needed for the full physiological activity of the tissues of a 

 mammal, while the, in many respects, similar tissues of a fish 

 work perfectly, although perhaps more sluggishly, at a much 

 lower temperature, is not quite clear ; nor do we know the 

 precise significance of that relative constancy of temperature in 

 the warm-blooded animal, which is as important and peculiar as 

 its absolute height. The higher animals must possess a superior 

 delicacy of organization, hardly revealed by structure, which 

 makes it necessary that they should be shielded from the shocks 

 and jars of varying temperature that less highly - endowed 

 organisms endure with impunity. Leaving the discussion of the 

 local differences and periodic variations of the temperature of 

 warm-blooded animals to a future page, let us consider now 



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