THERMAL PHENOMENA OF THE BODY 203 



brought in large quantities to the skin and correspondingly 

 drawn away from internal organs, and the evaporation of 

 perspiration becomes increasingly important as the external 

 temperature rises from 70 F. to 90 and 100 F. The 

 organism is striving against a rise of its body temperature. 



About 68 or 70 F., however, the situation changes ; for, 

 as the external temperature continues to fall, heat begins to 

 be transferred to surrounding objects more rapidly than it is 

 produced. The temperature of the body would fall if no 

 means were taken to prevent the result. Even during the 

 fall from 90 to 70 the cutaneous arterioles, widely dilated 

 at the higher temperature, have been gradually increasing 

 their tone and so sending diminishing quantities of blood 

 through the skin. Below 68 to 70 this tone rapidly increases ; 

 the veins are no longer conspicuous on the hand and arm; 

 if the blood is forced out of a portion of the skin by gentle 

 compression with the finger, the color returns slowly, indi- 

 cating considerable constriction of the cutaneous arterioles. 

 At the same time the arterioles of internal organs are dilat- 

 ing (see p. 152) so that the liver, the kidneys, the mucous 

 membranes of the alimentary canal and of the air passages 

 contain an increasing quantity of blood. The body is now 

 striving against a fall of its internal temperature by driving the 

 blood from the skin back upon internal organs. 



By the time the temperature has fallen to 60 F., or there- 

 abouts, the cutaneous arterioles have constricted to their 

 utmost, the blood flow through the skin has nearly ceased, 

 and the organism has no means at command by which to 

 restrict the further output of heat. If in this emergency 

 heat production were to remain constant while external tem- 

 perature continued to fall, the temperature of the body would 

 be lowered, for the transfer of heat would not only continue 

 but increase. That it is not usually lowered is due solely 

 to the fact that more heat is then produced within the body ; 

 the oxidations (and hence heat production) which have 



