NERVOUS CONDUCTION AND EXCITATION 143 



the nerve at the same rate as the propagated disturbance 

 itself. 



The first proposition implies that a means of treating the 

 intensity of the nervous impulse by quantitative methods is 

 available. Adrian (19 12) has sought to estabHsh this on the 

 assumption that the nervous impulse suffers decrement in 

 passing through a region of narcosis. If a nerve is narcotised 

 in a gas chamber, a stimulus applied within the region subject 

 to narcosis (inside the gas chamber) will evoke a response in 

 its attached muscle after response to stimulation on the far 

 side of the gas chamber has been abolished ; the depth of 

 narcosis, as measured by the time of exposure, required to 

 abolish response becomes continuously less as the length of 

 nerve exposed is increased. Thus the ability of the nervous 

 impulse to traverse a region of narcosis can be employed as 

 a measure of the strength of the impulse. By arranging 

 electrodes at intervals along the course of a nerve, enclosed 

 at intervals along its course by gas chambers in which 

 measurable lengths are subjected to narcosis, it can be shown 

 that, when a length of nerve is narcotised until its power of 

 conduction is almost abolished, it retains its ability to transmit 

 an impulse through a second region of narcosis of length just 

 sufficient to abolish response to a stimulus applied immediately 

 outside the latter. If the impulse is not completely 

 extinguished in passing through a region of decrement, it 

 recovers its full capacity to face exposure to the same degree 

 of narcosis ; on re-entering a normal region it regains its 

 original intensity. Nervous conduction is thus an all-or- 

 none phenomenon, i.e. its energy of propagation is independent 

 of the strength of the stimulus which initiates it, being dis- 

 tributed along the whole course of the neurone. 



To avoid undue abstraction an experiment of Adrian (19 12) 

 may be described in detail. Four gas chambers as in Fig. 35 

 are arranged. A and B have a diameter of 4*5 mm. The 

 diameter of C is 9*0 mm. Nerves are arranged for the experi- 

 ment as indicated. In all chambers the depth of narcosis 

 for the length of nerve traversed is identical. The narcotic 

 used by Adrian was alcohol vapour. In each experiment the 



