THE CONDUCTIVITY OF NERVE. 455 



extremely small, or is such that any chemical dissociation is rapidly 

 compensated for by subsequent association. 



The conductivity of nerve — Rate of transmission. — The pro- 

 pagation of the excitatory state along the nerve fibre, from the seat of 

 excitation, is probably the most fundamental fact in nerve physiology. 

 The experimental basis of the fact is the series of observations made by 

 Helmholtz ; these showed that, when the sciatic nerve of the frog was 

 excited at its central end, the period between such excitation and the 

 response evoked in the attached muscle was perceptibly longer than in 

 the case of excitation at the muscular end of the nerve. The period 

 of delay was increased in the former case by -001 sec. when, with the 

 nerve at 15° C, a distance of 27 mm. lay between the central and 

 peripheral seats of excitation. In order to ascertain the interval of 

 time between the excitation and the muscular response, Helmholtz 

 employed the galvanometric method of Pouillet. 1 The movement of 

 a suitable key caused an induced current to traverse and excite the 

 nerve, whilst it closed, at the same instant, a galvanic current of 

 known intensity through a circuit comprising a galvanometer. The 

 contraction of the muscle evoked by the arrival of the excitatory state 

 was utilised to break a contact in this galvanometer circuit, and 

 thus the duration of the galvanic current comprised the period 

 between the nerve excitation and the lift of the contact through the 

 muscle response. As the extent of the deflection of the needle was, 

 in the instrument used, proportional to the time of current flow, the 

 difference between this time with a central and a peripheral stimulation 

 could be readily ascertained. 



Similar experiments were carried out by Helmholtz on man, the 

 muscles in the ball of the thumb being used as the index, and the nerve 

 excited at the root of the neck and bend of the elbow respectively. 



The experiments have been repeated under many varying conditions, 

 the movement of the muscular response being recorded by any one of 

 the methods employed for the accurate determination of the period of 

 delay in the response of muscle to indirect or direct excitation. 



The transmission is undoubtedly due to each successive portion of 

 the continuous nerve fibre being thrown into the state of excitation, in 

 consequence of the production of this state in the neighbouring portion. 

 This view is confirmed by the determination of the rate of propagation 

 of the excitatory state itself, as evidenced by its accompanying electro- 

 motive changes. These are transmitted in the case of the sciatic nerve 

 of the frog at a rate which is similar to that deduced from the time- 

 relations of the muscle response. 2 



The propagation rate (27 metres per second) is slow compared with 

 that of sound, light, etc., but is much faster than exists in the case of 

 muscle. It differs in different classes of nerves, and may be altered by 

 a large number of agencies, which will be considered in the succeeding 

 sections. 



The alteration is one not merely of the rate but also of the intensity 

 of transmitted effect. Thus, on passing through a locally cooled region, 

 the transmission is both delayed in time and diminished as to intensity : 

 the resultant muscular response thus occurs later, and may be smaller 

 than it otherwise would be. 



1 Helmholtz, Arch. f. Anat., Physiol, u. vnssensch. Med., 1S50 and 1S52. 



* Bernstein, "Untersuch. u. d. Enegungsvorgang in Nerven u. Muskeln," 1S91. 



