in ORGANIC ADAPTABILITY 25 



ance of an almost even temperature by all warm-blooded 

 animals. In maiij the temperature of the blood is main- 

 tained, no matter what the temperature of the surrounding 

 air, at 98*4 F. The fluctuations around this point 

 during health are so narrow that any marked deviation is 

 at once taken as a symptom of illness. Without going 

 into detail, it is sufficient to say that this balance is main- 

 tained principally by two mechanisms, the vasomotor 

 system and the perspiration. The vasomotor nerves 

 govern the calibre of the small arteries which supply the 

 surface of the body with blood, dilating or constricting 

 them as the case may be. The application of something 

 cold to the skin acting through these nerves causes con- 

 striction of the blood-vessels. The result is that less 

 blood comes to the surface, and as it is the blood that 

 brings the heat the skin is cooled, but the body as a 

 whole gives off less heat than usual. Exactly the opposite 

 result follows from the application of heat. Now the 

 small arteries are dilated, the surface is flushed with 

 blood, and heat is rapidly lost. Similarly, the increase 

 of heat again acting through a nervous mechanism throws 

 the sweat glands into activity, and the consequent 

 evaporation keeps the body cool. 



" The working of this heat-regulating mechanism is well seen 

 in the case of exercise. Since every muscular contraction gives 

 rise to heat, exercise must increase for the time being the pro- 

 duction of heat ; yet the bodily temperature rarely rises so much 

 as a degree centigrade, if at all. By exercise the respiration is 

 quickened, and the loss of heat by the lungs increased. The 

 circulation of blood is also quickened, and the cutaneous vascular 

 areas becoming dilated, a larger amount of blood passes through 

 the skin. Added to this, the skin perspires freely. Thus a large 

 amount of heat is lost to the body, sufficient to neutralise the 

 addition caused by the muscular contraction, the increase which 

 the more rapid flow of blood through the abdominal organs might 

 tend to bring about being more than sufficiently counteracted by 

 their smaller supply for the time. The sense of warmth, which 

 is felt during exercise in consequence of the flushing of the skin 

 is in itself a token that a regulative cooling is being carried on! 

 In a similar way the application of external cold or heat defeats its 

 own ends, either partially or completely." 1 



1 Foster, op. cit. Part II. p. 849. 



