1902.] Photographic Records of the Response of Nerve. 



213 



i.e., it is a function of three variables, namely, dp/dt, the number of 

 linear conductors coming into action, dtr/dt, the growth of the difference 

 of potential at each point of each conductor, and dq/dt, the diminution 

 of the number of active conductors between the leads owing to some 

 of them not extending from P to A. Of these, dp/dt and dir/dt can 

 only be separated by such experimental methods as are tantamount to 

 changing the distribution of the points of origin. 



For instance, a muscle may be excited directly or by its nerve. In 

 the latter case, the distribution of the points of origin is conditioned 

 by that of the nerve endings — in the former case, by the disposition of 

 the exciting electrodes. By a comparison of the results, a determina- 

 tion of / (p) may be attempted, leaving, however, the uncertainty as 

 to whether the altered mode of excitation may or may not have 

 modified / (ir). 



What may be termed the centre of gravity of the points of origin 

 can easily be found on a gastrocnemius or sartorius by placing the leads 

 one above and one below the nerve entrance, and shifting them till no 

 excursion of the meniscus results on excitation. 



Duration. 



If the apex of the spike is very sharp and its beginning and end are 

 gradual the analysis will show in many cases that the transition from 

 maximum electro-positive to maximum electro-negative occupies about 

 as long as the development of the first or electro-positive phase from 

 zero. This is commonly met with in fresh muscle, especially gastroc- 

 nemius,* and fresh uninjured nerve. 



It indicates that the duration of the active condition at each point 

 is so related to rate of propagation, the development of the E.M.F. 

 and its subsidence, the distribution of the centres of origin, and the 

 distance between the leads, that the maximum slope of the wave-front 

 (to make use of the other mode of expressing the problem) just reaches 

 the electrode A as the maximum slope of the end of the wave passes B. 



In other words, a sharp apex results from a particular relation 

 between 0, v, p, k, tt, and q, and the leads A and B, so that a sharp 

 apex may become flat-topped from several quite different causes. 



Flat-topped apices may be given by — 



(1.) Kept nerves, especially at low temperatures. 

 (2.) Nerves which have been experimented on a great number of 

 times. 



(3.) Nerves under the action of COo. (See fig. 13.) 



(4.) With leads further apart. 



(5.) With leads closer together than usual. 



* See my paper on the " Time Relations of the Capillary Electrometer," 4 Phil. 

 Trans.,' A, vol. 183, ] 892, Plates 5 and 6. 



