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 
