ANIMAL HEAT 513 



measured by the amount of carbon dioxide given off, is more sensi- 

 tive to changes of external temperature than in the normal animal. 



But it must not be imagined that the production of heat can be 

 increased indefinitely to meet an increased heat-loss. The organism 

 can make considerable efforts to protect itself, but the loss of heat 

 may easily become so great that the increase of metabolism fails to 

 keep pace with it. The internal temperature then falls, and if the 

 fall be not checked, the animal dies. A mammal, when cooled arti- 

 ficially to the temperature of an ordinary room (15 to 20 C), does 

 not recover of itself, but may be revived by the employment of arti- 

 ficial respiration and hot baths, even when the rectal temperature 

 has sunk to 5 to 10 C If the skin of a rabbit be varnished, and 

 the air which it is the function of the fur to maintain at rest around it 

 be thus expelled, the animal dies of cold, unless the loss of heat is 

 artificially prevented. If, without varnishing at all, the greater portion 

 of the skin of a rabbit or guinea-pig be closely clipped or shaved, 

 similar phenomena are observed. Prevented from covering itself 

 with straw, the animal dies, sometimes in twenty-four hours. The 

 radiation from the skin, as measured by the resistance-radiometer 

 (p. 496), is greatly increased ; the animal shivers constantly, and the 

 rectal temperature falls. Placed in a warm chamber before the 

 temperature in the rectum has fallen below 25, the animal recovers 

 perfectly. If the fall is allowed to go on, it dies. If it is kept from 

 the first in the warm chamber, no fall of temperature occurs. When 

 the increased loss of heat is less perfectly compensated when, for 

 example, the animal is left at the ordinary temperature, but supplied 

 with sufficient straw to cover itself, or allowed to crouch among other 

 animals a curious phenomenon may sometimes be seen. The 

 rectal temperature, which has fallen sharply during the operation, 

 remains subnormal (as much as 2 to 3 below the ordinary tempera- 

 ture) for a time (a week or more), and then gradually rises as the 

 coat again begins to grow. The meaning of this seems to be that 

 the power of regulating the temperature by increasing the metabolism 

 is overtasked by the removal of the natural protective covering, 

 unless the escape of heat is artificially diminished. When the loss 

 of the fur is entirely compensated, no fall of temperature occurs ; 

 when it is not compensated at all, the animal cools till it dies ; when 

 it is partially compensated, the increased metabolism may only suffice 

 to maintain a temperature lower than the normal, although constant 

 muscular contractions (shivering) are brought in to supplement the 

 efforts of the regulative chemical processes. 



Hitherto we have only spoken of a reflex regulation of 

 the heat-production called into play by external cold. It 

 might be supposed and, indeed, has often been assumed 

 that heat would lessen the metabolism, as cold increases it ; 

 and there are indications that in the smaller animals this is 

 the case, although the influence of heat seems to be much 

 smaller than the influence of cold. But neither experi- 



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