THE ELECTRICAL RESPONSE OF NERVE. 529 



portionately weaker stimulus the nearer the excitation is to the 

 ligatured end of the knee branches, and thus to the more excitable end 

 of these nerves. Indeed, the response fails if the stimulus is shifted 

 nearer the muscles, and away from this ligatured cross section. It 

 thus fails if the stimulated nerve is not sufficiently excitable, and 

 it fails also if the excitability of the sciatic plexus is not raised by 

 previous cross section. The group of nerves in the plexus comprises 

 the upper extremities of the fibres which are being stimulated near the 

 knee, and also others which, lying in the plexus bundle, emerge lower 

 down to supply the adductor muscles. Although the two sets of fibres 

 are physiologically and structurally independent, excitation of the latter 

 is produced by the electrical responses occurring in the former. The 

 seat of this excitation is the combined nerve trunk of the plexus, the 

 excitability of whose fibres has been raised by its division, so that the 

 electrical response in the knee fibres, on arriving at the plexus, becomes 

 adequate to arouse other fibres lying in immediate proximity. The fact 

 that the rapid stimulation evokes the secondary tetanus, can only be 

 explained on the supposition that a series of electromotive disturbances 

 are produced and propagated up the primary nerve bundle, and that each 

 member of this rhythmical series is adequate to arouse the fibres of the 

 secondary nerve bundle. 



The negative variation caused by repeated stimulation thus com- 

 prises a series of electromotive disturbances, each member of the series 

 being transmitted along the nerve, and evoked by a single member of 

 the series of stimuli. The further consideration of the excitatory change 

 resolves itself into that of the electrical response evoked by a single 

 stimulus. 



The electrical response of nerve to a single stimulus. — For the 

 reasons already referred to, the simplest mode of investigating this is 

 one in which the electromotive difference between the surface and cross 

 section of a nerve is obtained, and any variation in consequence of 

 excitation observed. The variation is of such brief duration that gal- 

 vanometric indications of its presence can only be obtained by summing- 

 up a number of such variations by means of the repeating rheotome. 

 The capillary electrometer, if constructed so that the surface of the 

 mercury moves with very great rapidity, is, however, able to follow 

 the brief electromotive change. In order to observe the movement, 

 the surface of the mercurial meniscus is projected upon a travelling 

 sensitive plate, so that a quick movement of the magnified image may 

 be recorded, it being impossible to follow the rapid oscillation by the 

 eye. Under these conditions, an oscillation, indicating a rapid electro- 

 motive change, has been recorded by the capillary when the nerve is 

 excited by a single stimulus. 



The character of the recorded excursion is always such as to show 

 a rapidly developed change, in which the contact nearest the seat of 

 the stimulus becomes relatively negative to that further off. The 

 subsidence of this relative negativity of the tissue under the proximal 

 contact is greatly influenced by the condition of the nerve, but 

 when the distal contact is one involving a cross section of the 

 nerve, it is always somewhat prolonged. This is indicated in Fig. 

 270, in which the upward movement indicates negativity of the 

 proximal contact. As the moment of the stimulus is indicated on 

 the plate, the time relations of the electrical response to the single 

 vol. 11. — 34 



