674 ANIMAL HEAT 



sometimes in twenty-four hours. The radiation from the skin, as 

 measured by the resistance-radiometer (p. 657), 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 temperature) 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 sup- 

 posed and, indeed, has often been assumed that heat would lessen 

 the metabolism, as cold increases it; and ther^ are indications that 

 in the smaller animals this is the case, although the influence of 

 f heat seems to be much smaller than the influence of cold. But 

 neither experimental results nor general reasoning have as yet 

 shown that in man, either in the tropics (Eykman) or in the north 

 temperate zone (Loewy), the chemical tone is diminished by a rise 

 of external temperature much above the mean of an ordinary 

 English summer, apart from the effect of the muscular relaxation 

 which heat induces. In a man, indeed, at rest in a hot atmosphere, 

 the production of carbon dioxide and consumption of oxygen are, 

 if anything, greater than at the ordinary temperature. The regu- 

 lation of temperature in an environment warmer than the normal 

 seems, in fact, to be brought about more by an increase in the loss 

 than a decrease in the production of heat. Evaporation from the 

 skin and lungs is an automatic check upon overheating as important 

 as the involuntary increase of metabolism upon excessive cooling. 



Nervous Mechanism of Thermotaxis. While the skeletal 

 muscles, and perhaps the glands, are at one end of the reflex arc by 

 which the impulses pass that regulate the temperature through the 

 metabolism, we are as yet ignorant of the precise paths by which 

 the afferent impulses travel, of the nerve-centres to which they go, 

 and even of the end-organs in which they arise. There are nerves 

 in the skin which minister to the sensation of temperature 

 (Chap. XVIII.). A change of temperature is their ' adequate ' and 



