THE EXCITATION OF NERVE. 459 



Attention may now be drawn to the second condition for propaga- 

 tion, the susceptibility of the unexcited neighbouring portion. The 

 possession of this susceptibility, when the stimulating agent is an 

 external change, is the attribute of excitability ; when the stimulating 

 agent is the excitatory state itself in a neighbouring portion, it is 

 expressed as conductivity. It is evident that the two are phases of 

 one fundamental characteristic, but they are certainly more or less 

 independent phases of this. The evidence for such separation will be 

 furnished in detail in the sections dealing with the influence of various 

 conditions upon excitability and conductivity. 



The Excitation of Nerve. 



Modes of stimulation. — The term "excitability" is conveniently 

 limited to the possession by living nerve of a capability of respond- 

 ing by a definite change to the action of an external agent, and 

 every external agency which is capable of evoking such a response 

 is an efficient or adequate stimulus. Such "excitability" may 

 be measured in terms of the intensity of the minimal efficient 

 stimulus, if the exciting agent or stimulus is itself a measurable 

 one. The direct index of a nerve response is the electrical change, 

 which is the sole physical alteration at present ascertained in active 

 nerve, but an indirect index is afforded by the propagation of the 

 excitatory change from the seat of the local stimulus to such a peri- 

 pheral organ as a muscle, a contraction of which is evoked on the 

 arrival of the transmitted nerve effect. It is evident that this indirect 

 index is complicated by being the response of another structure, and 

 that all conclusions deduced from experimental investigations in which 

 this method is used, can only be made after full allowance for this. In 

 the great majority of the investigations upon nerve this muscular index 

 has been employed, and the results are thus, for the most part, confined 

 to motor nerves. The electrical response will be treated in the conclud- 

 ing chapters, so that in the present and immediately succeeding sections 

 the subject matter will be largely limited to the evidence afforded by 

 what may be termed the indirect index. In the case of afferent nerves, 

 a similar indirect index is the reaction of the central nervous system as 

 indicated either by reflex effects in peripheral organs, or by the 

 development of those changes which are transmuted in the crucible of 

 consciousness into sensation. A large variety of external agencies can 

 act as local stimuli for nerve fibres ; they may be grouped as electrical, 

 mechanical, chemical, and thermal. 



Of these the electrical stimulus is that principally employed on 

 account of the ease with which it can be applied to a definite region of 

 the nerve, the facilities its employment offers for graduating the 

 intensity of the stimulus, and the comparatively small amount of 

 injury which it causes in the tissue subjected to its action. 



Electrical stimulation. — (1) Make and break induced currents. — 

 The forms of electrical stimulus most frequently made use of are 

 the induced currents occurring in the secondary coil on the com- 

 mencement or the cessation of a current flow through the primary 

 coil of an induction machine. 



They are susceptible of very accurate graduation as to intensity by 

 altering the distance between the secondary and primary coils, and, in 



