ITS SCOPE AND METHOD 73 



to test the direction of the blood-flow along the vessels, 

 and by mechanical inferences from the form of the heart 

 and vessels, learned that the blood circulates throughout 

 the body, passing and repassing through the heart ; and 

 that the heart is a muscular pump which maintains the 

 circulation of the blood. 



To realize how the circulation equalizes the tempera- 

 ture through the body we have to remember that in 

 certain organs the blood tends to be warmed whereas 

 in others it is cooled. The blood that streams close 

 under the surface of the skin and lends that its rosy tint 

 is cooled in doing so. This on returning to the heart 

 and large blood vessels is mixed with warmer blood 

 from the rest of the body. Thus mixed it is redispersed 

 again from the heart to all parts both of the interior and 

 the skin. The circulation therefore diffuses the warmth 

 equably through the body. 



The thermometer shows that our surface temperature 

 not only differs from part to part, but that it varies from 

 time to time ; for instance, it rises on our entering a 

 warm room. But inside the body the temperature is 

 found hardly to vary at all. We may travel from the 

 pole to the equator without its varying. And in this 

 we resemble all warm-blooded animals, better called 

 animals of constant temperature. 



How is our body-temperature thus kept constant ? 

 The temperature of the blood as a whole will depend 

 on how large a fraction of it is cooled by streaming 

 through the skin. The regulation of that quantity is 

 a main means of regulating the deep temperature. It is 

 common experience that we flush after entering a warm 

 room. That is, the blood-vessels of the skin dilate. A 

 larger portion of the total blood streams through the 

 cool skin-zone of the body ; the escape of heat from 



