444 STUDIES IN ADVANCED PHYSIOLOGY. 



In such a simple way the experiment proved the rapidity 

 of a nervous impulse to be 28 meters; that is, a little over 

 92 feet per second. These, however, are the figures for the 

 motor nerves of the frog. The rate of transmission is some- 

 what faster in the motor nerves of warm-blooded animals, 

 and is here probably not far from 100 feet per second. 



Experiments on sensory nerves are, of course, not so 

 easy, but there is reason to believe that the speed of the 

 impulse is about the same as that of the motor nerves. 

 Such a speed of about 100 feet per second is, of course, 

 exceedingly slow compared with the rate of transmission of 

 an electric impulse, and would seem to settle conclusively 

 the difference between the two. 



That a nervous impulse is of a chemical nature seems 

 disproved by the fact that no exhaustion occurs in a nerve in 

 the transmission of this impulse. It has been possible in 

 experiments to send long continued repeated impulses along 

 a nerve without noticing at the close of the experiment that 

 the nerve had been thereby exhausted. Such stimulation of 

 course commonly exhausts the muscle with which the nerve 

 is connected, or if a sensory nerve the centers to which it 

 goes, but in a very slight degree, if any, the fiber itself. If 

 it were a chemical change it would be difficult to see how 

 an exhaustion caused by the chemical disintegration could 

 be avoided. The only explanation left, therefore, seems to 

 be that it is some kind of a molecular change which travels 

 along the fiber, a molecular change, however, of the nature 

 of which we are still wholly at sea. 



Several things about the impulse are known. Its rate 

 of speed is about 100 feet per second. We also know that 

 the nervous impulse is in the nature of a wave, which is 

 about 18 millimeters in length. This is a little over seven- 

 tenths of an inch. 



The wave-like nature of the impulse is evident from, 

 and can be measured by, a peculiar electrical wave which 

 runs along the nerve fiber with the nervous impulse, which 

 electrical wave is easily detected by means of the galvano- 



