700 ANIMAL HEAT 



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 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 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 Therm otaxis. 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 

 sufficient stimulus; and it is a tempting hypothesis that these are 

 the afferent nerves concerned in the reflex regulation of temperature 

 that impulses carried up by them to some centre or centres in 

 the brain or cord are reflected down the motor nerves to control 

 the metabolism of the skeletal muscles, and down the vaso-motor 

 nerves to control the loss of heat from the skin. 



It is more than doubtful, however, whether the whole chemical regu- 

 lation can be attributed to such stimuli. For it has been found that the 

 relation between heat-production and extent of surface in animals 



