THE PHYSIOLOGICAL EVOLUTION, ETC. 391 



mechanism which can bring about an increased metabolism 

 and heat' production of the muscles and other tissues of 

 the body, apart from actual muscular movements. Thus, if 

 these are controlled by an effort of the will, as in these 

 experiments on man, the body temperature seems to fall 

 appreciably ; whilst in the guineapig, where, of course, no 

 such voluntary control is exercised, we have seen that the 

 nervous system is unable to keep the body temperature 

 constant. In the long run, however, it appears that the 

 nervous heat regulating mechanism can perform its function 

 more efficiently, and that it is only when the animal is exposed 

 to somewhat rapid variations of external temperature that 

 it is much at fault. Thus, as the result of numerous observa- 

 tions by Davy, 1 Rattray,- Crombie 3 and others, it has been 

 found that the body temperature of man in the tropics is 

 only a fraction of a degree above that observed in temper- 

 ate zones. In arctic regions also, it suffers no diminution. 

 Under these circumstances, the body must obviously be 

 losing much less or much more heat than in temperate 

 zones, and yet the nervous system is able to counterbalance 

 the varying loss. 



We thus see that the warm-blooded animal is constituted 

 similarly to the cold-blooded one in some respects. Increase 

 of temperature does tend to send up the body temperature, 

 and to bring about an increased metabolism, but it is only 

 under somewhat exceptional circumstances that the counter- 

 balancing nervous influence is unable to more or less 

 effectually cope with this ever present tendency. Under 

 abnormal conditions the mechanism may be totally at fault. 

 Thus numerous instances are recorded of drunkards who 

 have been left exposed on cold nights, and in whom the 

 body temperature has fallen to as low as 24*0°, 247° and 

 25*0°, but with subsequent gradual recovery to the normal 

 temperature. 4 In cases of fever again, the heat regulating 

 mechanism is also found wanting ; in some cases, as of 



1 Phil. Trans., 1850, p. 437. 



- Proc. Roy. Soc, vol. xviii., p. 526, 1870. 



3 Indian Ann. Med. Soc, vol. xvi., p. 550, 1873. 



4 Vide Schafer's Text- Book of Physiology, vol. i., p. 821. 



