A CCO M MO DA TION 225 



maximum axon potential was approximately directly proportional to the 

 diameter of the active fiber. This conclusion is supported by theoretical 

 considerations. 



Whether the simple relation — the velocity of propagation of the 

 nerve impulse is directly proportional to the diameter of the nerve fiber 

 — can be extended to cover a much wider range of fiber diameters is still 

 an open question in view of the measurements by Pumphrey and Young 

 [1938]. These show that in cephalopod (squid) nerves, which contain 

 fibers varying in diameter from 30 to 700 microns, measurements of the 

 velocity of propagation were nearly proportional to the square root of the 

 diameter, and that the smaller nerves of vertebrates (1 to 20 microns) 

 gave values of velocity which were proportional to the diameter of the 

 fiber. 



The conduction properties of a nerve may be reduced by a drop in 

 temperature to such an extent that a local application of ice acts as a 

 nearly complete depressent to the nerve impulse as it passes through the 

 low-temperature nerve section. The velocity of propagation, however, 

 will be restored when the nerve regains its original temperature. 



Anesthetics and narcotics, such as ether, chloroform, cocain, chloral, 

 phenol, and alcohol, may be applied locally to a nerve trunk to decrease 

 the conductivity and irritability, which are restored when the narcotic 

 is removed. 



Accommodation 



When a stimulus in the form of an electric current is applied to a nerve 

 for a short time, the critical value of current strength to excite — that is, 

 the threshold value — is constant. Under these experimental condi- 

 tions the applied current builds up a local excitatory process which sets 

 the nerve impulse in motion when it reaches a critical value. If a stimu- 

 lating electric current, insufficiently large to excite, is removed from a 

 nerve, the excitatory process is assumed to revert to its resting value, 

 according to the law of exponential decay. 



If a subthreshold direct current is used as a stimulus and allowed to 

 increase slowly with time, it is found that the current may attain a very 

 high value without producing a response in the form of a moving nerve 

 pulse. This result is possible if the rate at which the excitatory process 

 decays is equal to, or greater than, its rate of increase. 



If a constant subthreshold direct current passes through a length of 

 nerve, the excitability of the nerve is increased at the cathode contact- 

 point and decreased at the place where the anode makes contact. If the 

 excitability at the cathode contact is examined as a function of time of 

 application of a stimulus (Bishop [1928]), it is found that the excitability 



