8 A CHEMICAL SIGN OF LIFE 



When it was found that an electrical change occurred 

 in a nerve when it conducted an impulse, the problem was 

 considered to be settled. The nerve impulse was sup- 

 posed to be electrical in nature. This idea was soon 

 questioned, however, when the speed of the conduction 

 of a nerve impulse was found to be so slow in comparison 

 with that of an electrical current. The speediest nerves, 

 such as those of human beings, conduct impulses only 

 at the rate of a hundred meters per second, whereas 

 electricity travels in a wire at a speed of thousands of 

 kilometers per second. One thing seemed to be certain- 

 that the nerve impulse can pass through a fiber without 

 consuming any material. It was found that some 

 nerves could not be fatigued even on prolonged stimula- 

 tion. This fact supported the idea that certain quickly 

 reversible physical conditions must exist in the nerve, and 

 that the changes in these conditions, rather than chemical 

 changes, must determine the phenomena of irritability 

 and conductivity. Ultimately physiologists settled 

 down to the view that the physical and fundamental 

 changes concerned in irritability were either a change 

 of colloidal state, of surface tension, or in the permea- 

 bility of the nerve to salt, or changes in the distribution 

 of electrically charged particles in the nerve. 



Although such physical changes as these in nerves 

 have never been demonstrated experimentally, biolo- 

 gists generally have tried to explain the nature of a 

 nerve impulse and the phenomena of excitation purely 

 on the basis of these hypothetical physical changes; 

 and they have neglected the chemical changes. They 

 have also attributed many other important physiological 

 functions, such as secretion and contractility, to these 



