THERMAL PHENOMENA OF THE BODY 



205 



In all cases, rest or muscular activity, high or low 

 humidity, wind or calm, wherever the point of optimum 

 external temperature may be, we always find above this 

 point the region of active measures for heat dissipation, 

 and below it the region of active heat production. This is 

 graphically shown in Fig. 88. 



18. The "danger zone " of atmospheric temperature. We have 

 seen that, as the temperature falls from 70 to 60, the main 

 agency employed for temperature regulation is the diminution 

 of the blood flow through the skin, with its compensating in- 

 crease of the, blood flow within internal organs, thereby retaining 



100 90 80 70 60 50' 40 



Rest, normal humidity, 



no wind 

 Rest, high humidity 



Rest, wind 

 Muscular activity 



I = Optimum temperature 



I = Point at which increased heat production begins 



Blank space between the two indicates region of the " danger zone" 



FIG. 88. Variations in the optimum temperature 



as far as possible the heat within the body. This threatens 

 serious congestions and other unhealthful conditions, which 

 we shall consider at length in our study of hygiene (see 

 Chap. XXI). It is because the temperature of a room may 

 fall from 66 to 60 so gradually that we do not notice it 

 until the internal damage is done, whereas it could not fall 

 to 50 or 40 without our noticing it and correcting the 

 trouble, that more colds are taken in the former case than 

 in the latter. In other words, as the temperature goes below 

 65 the body seems at first to rely wholly on the vascular 

 mechanism of temperature regulation, and does not begin to 

 produce more heat until this resource has been utilized not 

 only to its utmost, but even to an extent inconsistent with 

 health. The "danger zone" temperature may then be defined 



