226 THE INTELLIGENCE SERVICE 



is due to differences in the structure and diameter of fibres. The 

 former has been referred to on p. 222 ; as regards the latter, we 

 may say that velocity is a linear function of the diameter of similar 

 fibres. The rate of transmission also alters with the temperature 

 level, as does the duration of the local electrical disturbance. 



8. Cause of the electrical change. It is generally admitted that 

 the action potential difference is the sign of a local ionic change in 

 nature similar to that occurring in other protoplasmic units in 

 action. In the first place, let us consider why the electrical change 

 is local. That is, the nerve fibre is not to be looked upon as a simple 

 electrical conductor, but rather as a chain of membrane-bounded 

 chambers containing ionised protein, ionised salts, etc., and 

 developing a local Donnan membrane -potential (cf. muscle 

 fibrillae). When one of these chambers is " excited " there is a 

 heaping up of ions on the membrane separating it from the next 

 chamber in order of progression, and this in some ways leads to 

 an alteration in permeability— an injury of the membrane, and the 

 disturbance passes to the next chamber, and so on. The evidence 

 for the local nature of the change is primarily twofold. 



(a) Resistance. The resistance of the nerve fibre to the conduc- 

 tion of an electric current is about 10'^ ohms per centimetre, and 

 varies with the cross-section of the fibre. According to Ohm's law, 

 the current flowing in a conductor varies directly as the potential 

 difference (E), and inversely as resistance (R) of the conductor, 



E 



i,e. C = p" If, for example, the value of 1 volt be given to E, 



then the current flowing in a fibre 10 nun. long would be 10"** 

 amperes. We have no means at present of computing the E.M.F. 

 developed in nerve, but it would need to be well beyond physio- 

 logical limits to produce a current flow in, say, the sensory nerve 

 fibre bringing the sensation of tickle from the sole of the foot. 



{h) Decrement of the nervous impulse. If a length of nerve is 

 cooled, not only does the velocity of the propagation of the 

 impulse suffer diminution, but there seems to be a diminution in 

 the intensity of the impulse as well. If the degree of cold is 

 sufficient, or if the length cooled is extensive, the impulse may be 

 stopped entirely. If, however, any of the impulse is propagated 

 through the cooled region into a normal piece of nerve it seems to 

 recover its full intensity and velocity. Lucas compares this 

 phenomenon to the transmission of fire along a fuse of gunpowder. 

 If a section of the fuse is slightly damp, the rate of burning as 

 well as the heat evolved will be decreased. l)ut will recover as 

 soon as combustion starts on a dry section. Narcotisation of a 

 nerve by ether, alcohol, cocaine or other drug has a similar effect 



