368 FRAGMENTS OF SCIENCE 



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 ad- 

 mitted 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 ac- 

 tion 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 themselves 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 effer- 

 ent nerves. The former carry impressions from the ex- 

 ternal 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 ani- 

 mal 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 lucid- 



