APPARATUS FOR PHYSIOLOGICAL USE. 253 



muscle was not dependent upon the lungs, the blood, or 

 anything contained in the rest of the body for its power 

 to contract, but contained in itself all that was necessary 

 to convert chemical into mechanical energy. If you had 

 put that very muscle into a test tube, placed it over 

 mercury, and analysed the gases given off from it, you 

 would have found that, although separated from the body, it 

 gave off carbonic acid, just as the whole body did, and this 

 carbonic acid would have been increased if you had made 

 the muscle contract. The muscle gives off more carbonic 

 acid during the period of activity than it does during the 

 period of rest. Combustion then goes on within the muscle 

 itself. But this combustion does not go on exactly in the 

 same way in the muscle as it does in the steam-engine, be- 

 cause if you entirely cut off the supply of air from the furnace 

 of a steam-engine the fire will go out, but if you had put that 

 muscle into a vacuum it would still have contracted, and 

 would still have given off carbonic acid. There is, therefore, 

 a likeness between the muscle and the steam-engine, inas- 

 much as they both produce motion by means of combustion, 

 but they differ in the way in which the combustion goes 

 on within them. In this respect, indeed, the muscle re- 

 sembles a gun rather than a steam-engine, for it lays up 

 within itself a store of oxygen by which it can keep up com- 

 bustion for some time, just as gunpowder has oxygen stored 

 up in its nitre, by which its charcoal is burned within 

 the closed tube of the gun, yielding carbonic acid, and 

 causing an explosion. 



Indeed, the contractions of a muscle have been very 

 fairly compared to a series of very small explosions, and 

 each contraction may be regarded as a single explosion. 

 But the muscle would soon use up all its store of oxygen, 

 and it would cease to contract if fresh oxygen were not 

 supplied to it. Yet how is the muscle to obtain oxygen, 

 for it is a long way removed from the external air which is 

 the only source of oxygen for animals ? It obtains it by 

 means of the blood, which conveys the oxygen to the 

 muscles from the external air with which it comes into close 

 relation in the lungs. 



Blood will dissolve oxygen like any other fluid, but the 

 quantity of oxygen which it could carry dissolved in this 

 way to the muscles would be insufficient for their working, 



