38 SCIENCE PRIMERS, [§ in. 



of the blood. Those things going to the 

 muscle give it strength and enable it to 

 contract. And that is why food makes you strong. 

 20. But you are always wanting food day by day, from 

 time to time. Why is that ? Because the muscle in 

 getting strength out of the food changes it, uses it 

 up, and so is always wanting fresh blood and new food. 

 We have seen in Art. i that food is fuel. We have 

 also seen that muscle (and other parts of the body 

 do the same) is always burning, burning without flame 

 but with heat, burning slowly but burning all the same, 

 and doing the more work the more it bums. The 

 fuel it bums is not dry wood or coal, but wet, watery 

 blood, a special kind of fuel prepared for its private use, 

 in the workshop of the stomach or elsewhere, out of 

 the food eaten by the mouth. This it is always using 

 up ; of this it must always have a proper supply, if 

 it is to go on working. Hence there must always be 

 fresh blood preparing ; hence there must from time to 

 time be fresh supplies of food out of which to manu- 

 facture fresh blood. 



To understand then fully what happens when you 

 bend your arm, we have to learn not only what we 

 have learnt about the bones and the joint and the 

 muscle and the nerves, about the machinery and the 

 engine, we have to study also how the food is changed 

 Into blood, how the blood is brought to the muscle, 

 what it is in the blood on which the muscle lives, what 

 it' is which the muscle bums, and how the things 

 which result from the burning, the ashes and the 

 smoke or carbonic acid and the rest of them, are 

 carried away from the muscle and out of the body. 



Meanwhile let me remind you that for the sake of 



