148 Br. J. Bur don- Sanderson [June 9, 



brain becomes cognisant of wliat goes on at the surface of the body 

 cannot be attributed to the communication of any visible or sensible 

 motion. Newton, although at that time no one had ever seen nerve- 

 fibres as we now see them under the microscope, yet described 

 them with perfect truth as " pellucid and uniform hair-like fila- 

 ments," in which vibratory motion could be propagated.* This con- 

 ception Haller, who was certainly not wanting in imagination, 

 rejected. Failing to understand that the thrills which Newton con- 

 templated were of an order far more subtile than those of sound, he 

 argued that, if the function of nerve were dependent on the propaga- 

 tion of vibratory motion, these would so interfere one with another, 

 that all distinctness of impression and of action would be lost, &c, 

 Haller's doctrine of the nerve-fluid held undisputed sway for a 

 century ; we have still traces of it in the language used by medical 

 writers. But the notions which we now entertain on nerve-function 

 are much more allied to those of Newton — so like, indeed, that they 

 might be clothed in his language. 



The transmission of an impression, i. e. of a state of excitation in 

 a nerve, has been justly comi^arcd to the propagation of a mechanical 

 disturbance along a row of card houses, so arranged that the collapse 

 of any one of them necessarily determines that of its neighbours. 

 Such a structure exhibits the properties assigned by Newton to the 

 pellucid capillamenta of nerves, its card houses corresponding to the 

 particles of which the pellucid substance was conceived by him to be 

 made up. In the one case, as in the other, a disturbance (excitation) 

 which originates at any point is propagated in either direction, reaching 

 its goal in a time which is proportional to the distance travelled. 



A still better illustration is furnished by the propagation of an ex- 

 plosion. Here, for example, is a train of gun-cotton. "j" When I excite 

 the end of the nerve with a match, a blaze runs along to the opposite 

 end, of which you can easily trace the progress, and if, in repeating the 

 experiment, I partially block the explosion by compressing the strand 

 midway by a weight, you see plainly enough that propagation is 

 retarded at the obstacle. 



The main ground for the statement I have made to you, that the 

 transmission of an impression along a nerve is analogous to the pro- 

 pagation of an explosion, lies in the proof first given by Helmholtz, 

 that time is lost in the transmission of excitatory effects along nerves, 

 and that the time is j)i'oportional to the distance. This I will 

 endeavour to illustrate by an experiment. Tracing the motor-nerve 

 channels by which, in the human body, the influence of the will is 

 conveyed to the muscles of the thumb and finger as in the act of 



* See Query 24 at the end of the third Book of Newton's Optics, Dr. Horsley's 

 edition, 1782, p. 226. 



t Mr. Abel, with the greatest kindness, enabled me to illustrate this part of 

 the lecture by experiments, showing how, according to the nature of the ex- 

 plosive substance, the velocity of propagation is very different, though the mode 

 of propagation is the same. 



