90 



AN AMERICAN TENT-BOOK OF PHYSIOLOGY. 



excited in two succeeding experiments at two points, at a known distance apart, 

 and tin difference in the time records obtained was the time required for the 

 transmission of the nerve-impulse through this distance. 



Fig. 35. — Method of estimating rate of conduction in motor nerve of frog, as vised by Helmholtz. The 

 horizontal bara?> is supported on an axis in such a manner that when the contact is made at a it is 

 broken at b, therefore at the same instant a current is made in the galvanometer circuit g and opened in 

 tin- primary circuit of the induction apparatus /). When the muscle contracts, the galvanometer circuit 

 is broken at c. The nerve was stimulated in two successive experiments at d and e. 



Later, Helmholtz devised a method by which a muscle would record its 

 contractions on a rapidly moving surface, and employed this to measure the 

 rate of conduction in motor nerves. He stimulated the nerve as near as 

 possible to the muscle and let the contraction be recorded; then he stimulated 

 the nerve as far as possible from the muscle, and again had the contraction 

 recorded. The difference in time between the moment of excitation and the 

 beginning of the contraction in the two experiments was due to the difference 

 in the distance that the nerve-impulse had to pass in the two cases, and, this 

 distance being known, the rate of conduction could be readily calculated. 

 By this means he found the rate of transmission in the motor nerves of the 

 frog to be 27 meters per second. In similar experiments upon men he 

 recorded the contractions of the muscles of the ball of the thumb, and noted 

 the difference in the time of the beginning of the contractions when the 

 median nerve was excited through the skin at two different places. He 

 found the average normal rate for man to be about 1)4 meters per second, a 

 rate which is considerably quicker than that of our fastest express trains, 

 but ;i million times less than the rate at which an electric current is trans- 

 mitted along a wire. These determinations are still accepted as approxi- 

 mately correct for human nerves, although they are found to vary very con- 

 siderably under different conditions, a high temperature and strong irritation 

 quickening the rate to 90 or more meters per second, while cooling may 

 gradually -low the rate and finally stop conduction. Moreover, considerable 

 differences exist in nerves controlling different functions, even in the same 

 animal. Thus Chauveau gives the rate for the fibres of the vagus nerve, 

 which supply the rapidly contracting striated muscles of the larynx, as 66.7 

 meters per second ; and the rate for vagus fibres, controlling the slower 

 smooth muscles of the (esophagus, as 8.2 meters per second. The rate of 

 transmission in the non-medullated nerves of invertebrates appears to be still 



