ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 443 



impulse carried by the optic nerve when this nerve is elec- 

 trically stimulated. 



Having now seen the oneness of all nervous impulses 

 the question arises, what is its nature? 



The Nature of a Nervous Impulse. 



Soon after the electrical current became known many 

 attempts were made by the older physiologists to explain 

 nervous impulses in terms of electricity. The analogy 

 between the nerves of the body and a system of telephone 

 or telegraph wires was too striking to be overlooked. But 

 all attempts to explain one in terms of the other have so 

 far been a failure. That a nervous impulse is not of an 

 electrical nature is evident for several reasons: A nervous 

 impulse will not travel along a dead nerve, or even a nerve 

 which has been numbed by cold. A nervous impulse will 

 not pass across a cut in a nerve, even though the two cut 

 ends be fastened together. Surely if nervous impulses were 

 of an electrical nature they would still pass in spite of these 

 difficulties. It has been possible to measure the rate at 

 which nervous impulses move. This measurement was first 

 accomplished by the physiologist Helmholtz. The experi- 

 ment is simple enough. A muscle was cut out from the 

 body of an animal and a long portion of the nerve leading 

 to it left intact. The muscle and nerve preparation were so 

 arranged that the time could be very accurately measured 

 between the moment when the nerve was stimulated and the 

 moment when the muscle contracted. The nerve was now 

 stimulated close to its insertion in the muscle, and the time 

 that elapsed between the stimulation of the nerve and the 

 contraction of the muscle carefully observed. Next, the 

 nerve was stimulated at its further end, that is, the nervous 

 impulse had now to go further through the nerve than it did 

 in the first instance. Again the time elapsing between the 

 moment of stimulation and the moment of contraction was 

 noted. The difference in time was of course the extra time 

 needed in the second case for the impulse to traverse the 

 added length of nerve. 



