6(8 FRAGMENTS OF SCIENCE. 



considered a mere heat-producer, into the motive power of 

 the organism. Mayer's prevision has been justified by 

 events, for the scientific world is now upon his side. 



We place, then, food in our stomachs as so much com- 

 bustible matter. It is first dissolved by purely chemical 

 processes, and the nutritive fluid is poured into the blood. 

 Here it comes into contact with atmospheric oxygen 

 admitted by the lungs. It unites with the oxygen as 

 wood or coal might unite with it in a furnace. The 

 matter-products of the union, if I may use the term, are 

 the same in both cases, viz., carbonic acid and water. 

 The force-products are also the same heat within the 

 body, or heat and work outside the body. Thus far every 

 action of the organism belongs to the domain either of 

 physics or of chemistry. But you saw me contract the 

 muscle of my arm. What enabled me to do so? Was it or 

 was it not the direct action of my will? The answer is, 

 the action of the will is mediate, not direct. Over and 

 above the muscles the human organism is provided with 

 long whitish filaments of medullary matter, which issue 

 from the spinal column, being connected by it on the one 

 side with the brain, and on the other side losing them- 

 selves in the muscles. Those filaments or cords are the 

 nerves, which you know are divided into two kinds, sensor 

 and motor, or, if you like the terms better, afferent and 

 efferent nerves. The former carry impressions from the 

 external world to the brain; the latter convey the behests 

 of the brain to the muscles. Here, as elsewhere, we find 

 ourselves aided by the sagacity of Mayer, who was the first 

 clearly to formulate the part played by the nerves in the 

 organism. Mayer saw that neither nerves nor brain, nor 

 both together, possessed the energy necessary to animal 

 motion; but he also saw that the nerve could lift a latch 

 and open a door, by which floods of energy are let, loose. 

 "As an engineer," he says with admirable lucidity, " by 

 the motion of his finger in opening a valve or loosening a 

 detent can liberate an amount of mechanical energy almost 

 infinite compared with its exciting cause; so the nerves, 

 acting on the muscles, can unlock an amount of power out 

 of all proportion to the work done by the nerves them- 

 selves." The nerves, according to Mayer, pull the trigger, 

 but the gunpowder which they ignite is stored in the 

 muscles. This is the view now universally entertained. 



