SCIENCE AND MAN. 351 



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 efferent nerves. The former carry impres- 

 sions 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 engi- 

 neer,' 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 themselves.' The nerves, according to Mayer, 

 pull the trigger, but the gunpowder which they ignite 

 is stored in the muscles. This ia the view now univer- 

 sally entertained. 



The quickness of thought has passed into a proverb, 

 and the notion that any measurable time elapsed be- 

 tween the infliction of a wound and the feeling of the 

 injury would have been rejected as preposterous thirty 

 years ago. Nervous impressions, notwithstanding the 

 results of Haller, were thought to be transmitted, if not 

 instantaneously, at all events with the rapidity of elec- 

 tricity. Hence, when Helmholtz, in 1851, affirmed, as 

 the result of experiment, nervous transmission to be a 

 comparatively sluggish process, very few believed him. 

 His experiments may now be mada in the lecture-room. 



