608 v FRAGMENTS or 
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. 
