8o6 THE POPULAR SCIENCE MONTHLY. 



the will sends down a sort of electric spark ' to the muscle ; and that 

 this spark, lighting up the explosive nitrogen, causes an immediate 

 union of the oxygen with the constituents of muscle, and so produces 

 the visible movement. 



Of course, voluntary actions, like automatic ones, liberate heat ; but 

 this heat is generally somewhat in excess of what is required for com- 

 fort, especially in hot weather. Lower animals, however, which have 

 no fires and no artificial clothing, require it more than we do to keep 

 us warm ; and even we ourselves in wintry weather always feel chilly 

 in the morning until we have had a good brisk walk to set up oxidation, 

 and consequently liberate enough heat to make us comfortable. 



Thus all motion, in the animal as in the steam-engine, depends upon 

 the union of oxygen with food or body-fuel. It is true that in the ani- 

 mal body oxygen can unite directly with carbon and hydrogen without 

 the necessity of a high temper9,ture, which we saw was indispensable in 

 the case of the coal, in order to bring the two sets of atoms within the 

 sphere of their mutual attractions. But the difference is probably due 

 to the diff'erent condition of the hydrocarbonaceous substances within 

 the animal body ; or else, as others conjecture, to the assumption by 

 the oxygen of that peculiar state in which it is known as ozone. At 

 any rate, the two processes do not disagree in any essential particular, 

 being both cases in which free substances, possessing dormant energy 

 by virtue of their separation and their affinity for one another, unite to- 

 gether, and in so doing liberate their energy as heat and visible motion. 



There is, however, one important distinction of detail between the 

 mechanism of a steam-engine and the mechanism of an animal body, 

 which gives rise to many of the mistaken notions as to the use of food 

 which we noticed above. In the engine, we put all the coal into the 

 furnace, and burn it there at once ; while the piston, cylinder, cranks, 

 and wheels are not composed of combustible material, but of solid iron. 

 In the animal body, on the other hand, every muscle is at once furnace, 

 boiler, and piston ; it consists of combustible materials, which unite 

 with oxygen in the tissues themselves, and set up motion "within the 

 muscle of which they form a portion. The case is just the same as 

 though the joints of an engine, instead of being quite rigid, w^ere com- 

 posed of hollow India-rubber and whalebone, with iron attachments ; 

 were then filled with coal, oxygen, and water, and possessed the power 

 of burning up these materials internally and setting up motions in the 

 India-rubber tubings. Hence the materials in the muscles are always 

 undergoing change. The carbon and hydrogen which have united with 

 the oxygen are perpetually forming carbonic acid and water ; and, as 



' I am speaking quite raetaphorically and popularly, and do not mean to imply adhe- 

 sion to the electrical rather than to the isomeric theory of nervous conduction. 



^ I purposely simplify and omit details, so as to give the reader a graphic and com- 

 prehensive picture of the central facts. So long as essentials are not distorted, a good 

 diagram is far better for educational purposes than an accurate facsimile. 



