TRA^■SMISSION OF THE EXCITEMENT. 115 



irritation and the pulsation of the muscle in the latter 

 case than in the former ; and this difference evidently 

 depends only on the ftict that in the latter case the 

 excitement within the nerve had to traverse a longer 

 distance, and therefore reached the muscle later, so 

 that the pulsation did not begin till later. 



This time may be measured, if the rate at which 

 the plate moved is known ; or if simultaneously with 

 the muscle-pulsation the vibrations of a tuning-fork 

 are allowed to indicate themselves on the plate. From 

 the time thus found and from the known distance 

 between the two irritated points of the nerve, the rate 

 at which the excitement propagates itself along the 

 nerve may be calculated. Helmholtz, on the ground 

 of his experiments with the nerves of frogs, found it to 

 be about 24 m. per second. It is not, however, quite 

 constant, but varies with the temperature, being greater 

 in higher and less in lower temperatures. It has also 

 been determined in the case of man. If the wires of 

 the inductive apparatus are placed on the uninjured 

 human skin, it is possible, as the skin is not an isolator, 

 to excite the underlying nerves, especially where they 

 are superficially situated. On thus irritating two points 

 in the course of the same nerve, the resulting pheno- 

 mena are exactly the same as those just observed in the 

 case of the nerves of frogs. In order to determine the 

 commencement of the muscle pulsation in the un- 

 injured human muscle, a light lever is placed on the 

 muscle in such a way that it is raised by the thickening 

 of the latter. Experiments of this kind were made by 

 Helmholtz with the muscles of the thumb. The appro- 

 priate nerve (}i. medianus) may be irritated near the 

 wrist and near the elbow. From the result inq- difference 



