THERMAL PHENOMENA OF THE BODY 207 



indicated lest we make the strain too great. It is a practical 

 point to remember in this connection that some forms of 

 muscular exertion introduce conditions for getting rid of the 

 surplus heat much more readily than others; this is especially 

 true of those which involve movement of the body as a 

 whole. Bicycle or horseback riding, by creating a breeze, 

 renders the cooling of the body a much easier matter than 

 does sawing wood, or swinging Indian clubs, or gymnastic 

 work in general; again, a particular form of exercise on a 

 dry day, when the perspiration can evaporate readily, may 

 be safe, while it would be decidedly inadvisable on a muggy 

 day, even though the temperature were somewhat lower. 

 Indeed, by this time the student must have learned that 

 the thermometer alone is no safe indicator of the difficulty 

 of heat elimination in warm weather. 



21. Relations of climatic conditions to mental work and 

 sleep. During mental work the brain requires an increased 

 supply of blood, and this is obtained partly by diminishing 

 the supply to the skin (constriction of cutaneous arteries) ; 

 during sleep, on the other hand, the supply to the brain is 

 diminished, and this is ordinarily effected by dilating the 

 arteries of the skin (see p. 155). Mental work is difficult 

 on very warm days, partly because it is difficult to bring 

 about cutaneous constriction ; and it is especially difficult 

 on warm, muggy days, since the maintenance of the constant 

 temperature then requires an excessive cutaneous dilation, and 

 the brain is quite unable to command its needed blood supply. 



It is also clear that since the arterioles of the skin should 

 dilate during sleep, and since they cannot readily do this 

 when the skin is exposed to cold, to " sleep warm " is good 

 advice, based on sound physiological principles. 



22. Digestion and the maintenance of the constant temper- 

 ature. During digestion, and especially during its earlier 

 stages, when secretion is at its maximum, a large supply 

 of blood is needed in the stomach, the pancreas, and the 



