106 PHYSIOLOGY. 



saturate with hydrochloric acid the one in the funnel representing the 

 capillaries of the lungs. 



Now, on working the heart the liquid will change from red to blue 

 in the funnel representing the body, and from blue to red in the funnel 

 representing the lungs. 



<; Anatomically there are two lungs, and the heart lies between them ; 

 physiologically, the lungs form a single organ, which is interposed be- 

 tween the two hearts." WILDER. 



The Changes in the Blood. What does the blood do 

 with the oxygen that it gets in the lungs, and where did it 

 get the carbon dioxid and other impurities that it brings 

 to the lungs ? Let us follow the blood and see. From 

 the pulmonary veins the blood goes to the left heart, and 

 is pumped to all the tissues except the lungs. Let us 

 follow a branch of the aorta that leads to a muscle. 



The Production of Heat and Motion in the Body. 



When a muscle works it becomes warmer. This has been 

 repeatedly proved by experiment. We know that we feel 

 warmer when we exercise. We know that the blood is 

 flowing more rapidly through the muscle when it is at 

 work. This more rapid stream brings the muscle more 

 oxygen. This it needs, for the heat of the muscle is pro- 

 duced by the oxidation of substance in the muscle. We 

 have seen that the oxidation of iron produces heat, and it 

 is the oxidation of the materials in the candle that enable 

 it to give out heat. But our bodies do not give out the 

 intense heat of a burning candle, nor do they produce 

 light, as is the case with the oxidation of iron and magne- 

 sium when those metals are burned. The slow oxidation 

 of the metals, in the presence of moisture, is more like 

 the oxidations in our bodies. It is by the oxidations of 

 the muscle (or substance in it) that the muscles produce 

 heat and that form of energy which gives motion. In the 



