132 HEAT AND MOTION. 



AY hat does the blood do with the oxygen that it gets in the 

 lungs ? and where did it get the carbon dioxid and other im- 

 purities 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 flow- 

 ing 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 produced 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 enables 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 oxi- 

 dation of iron and magnesium 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 (com- 

 paratively slow) ozidations of the muscle (or substance in it) 

 that the muscles produce heat, and that form of energy which 

 gives motion. In the case of the rusting of the metals there 

 is as much heat produced as when they are burned ; but the 

 heat is so slowly generated that it is given off about as fast 

 as it is produced, and we do not notice it. The oxidation 

 produces the waste matters, just as the burning of the various 

 substances produces waste. 



In our experiments with oxygen we see that substances 

 which burn in air will burn still more actively in oxygen. 

 But we must not infer from this that in our bodies the oxida- 

 tion of the tissues would be faster in pure oxygen. This is 



