SUMMARY.] PHYSIOLOGY. 129 



arm, it does work. The power to do that work, the 

 muscle got from the blood, and the blood from the 

 food. All the work of which we are capable comes, 

 then, from our food, from the oxidation of our food-, 

 just as the power of the steam-engine comes from the 

 oxidation of its fuel. But you know that in the steam- 

 engine only a very small part of the power, or energy, 

 as it is called, of the fuel goes to move the wheel. 

 Bv far the greater part is lost in heat. So it is with 

 our bodies : all the force we can exert with our bodies 

 is but a small part of the power of our food ; all the 

 rest goes to keep us warm. ..^ * 



Visiting all parts of the body, rebuilding and re- 

 freshing every spot it touches, the blood current also 

 carries away from each organ the waste matters of 

 which that organ has no longer any usei Just as each 

 part or organ has different properties and different 

 work, so also is the waste of each not exactly the 

 same, though all are alike inasmuch as they are all 

 the results of oxidation. The waste of the muscle 

 is not exactly the same as the waste of the brain or 

 of the liver. Possibly the waste things which the 

 blood bears from one organ may be useful to another, 

 and so be made to do double work, just as the tar 

 which the gasworks thr«w away makes the fortune of 

 the colour manufacturer. 



Be this as it may, the waste products of all parts, 

 travelling hither and thither in the body, come at 

 last to be brought down to very simple things, with all 

 their virtue gone out of them, with all, or all but all, 

 their power of burning lost, fit for nothing but to be 

 cast away, come at last to be urea or ammonia, car- 

 bonic acid, and salts. In this shape, the food, after a 



