THE ANIMAL A MACHINE 189 



it. In the daily rush of our lives few of us find leisure to 

 listen attentively to its pulse, to explore the secret of its 

 actions. 



Let us embark on another journey of exploration, this 

 time into the wonderland of the electrical actions which 

 occur in the animal body. Here we hope to find a clue 

 leading to an understanding of the nature of the nerve 

 impulse. What is that strange agent that travels along 

 the nerve fiber and after reaching the muscle elicits a con- 

 traction? After more than a century of investigation, 

 much evidence has now accumulated to show that it is 

 electrical in nature. It may seem natural to consider a 

 nerve as a kind of electrical telegraph wire, sending mes- 

 sages from the brain to the muscles and other parts of the 

 body. But such a comparison is hardly more than a play 

 on words. On closer inspection it appears that a nerve and 

 an electrical telegraph wire are radically different. The 

 distinction manifests itself in the following points: 



1. The wire consists of a metal, usually copper, which 

 is an excellent conductor of the electrical current; the 

 nerve fiber contains some sort of fatty material which is 

 insulating. 



2. An electrical current requires a closed circuit; hence 

 two wires are needed to connect an object through which 

 the current flows; in some cases the second wire may be 

 replaced by the moist ground, by a system of conducting 

 water pipes, by the rails in the case of electric locomotives, 

 but there are always two conductors. 



In the nerve-fiber, there is only one connection. 



3. But more than all that: the propagation of the current 

 along a wire occurs with the velocity of light, 186,000 miles 

 per second, an incomprehensible speed. 



