PHYSIOLOGY OF NERVE 277 



comparatively favourable object, it is probable that Maxwell's 

 mean value of 1'78 is sufficiently near the truth, and Keith Lucas ( 58 ) 

 as a result of observations by a different method on the frog gives 

 a mean value of 1'79. 



This is definitely higher than the usual physical value of 1'2, 

 and might be taken as indicating a chemical basis of nerve con- 

 duction if one could be certaip (which is very far from being the 

 case) that a tissue which contains loose combinations of electro- 

 lytes with proteins would not have a similar coefficient. At any 

 rate Keith Lucas shows that the value 1'78 to 2'01 is very nearly 

 the same for conduction in both nerve and muscle, and markedly 

 different from that of 3 '26 to 3'6 for the refractory period in 

 both tissues ( 57> 58 ), so that the underlying processes are probably 

 different in the two cases. 



Maxwell makes the suggestion that if nervous impulse is a 

 chemical process it is probably not a direct oxidation, as if it were 

 the coefficient would be higher. This would agree with the absence 

 of heat produced. But we can hardly pursue the subject further 

 in the present state of our knowledge, though it presents interesting 

 problems that further work may solve. 



II. Concentration Cells. Macdonald ( 59 ) has adopted this theory 

 of the causation of the nervous phenomena, and as he has based 

 his views on a long series of ingenious experiments, they merit at 

 least a careful examination, even if his conclusions are not accepted 

 in every detail. Adopting the same plan as before, and separating 

 experimental facts from the inferences therefrom, Macdonald's 

 work may be summarised as follows : 



(1) The specific resistance of nerve (sciatic of cat) is 180 ohms, 

 approximately equal to '3 per cent. NaCl. 



(2) When a nerve is placed for a short time in a dilute 

 solution of electrolytes, the injury current is increased ; in a strong 

 solution it is diminished. In the particular case of chlorides this 

 alteration follows the " concentration law," namely 



E,=Exlog 



where E to = the final E.M.F. after immersion. 



E = the initial E.M.F. before immersion. 



n = the concentration of the solution in gram-molecules per litre multiplied 

 by the dissociation coefficient. 



(3) The inference is drawn from these and other considera- 



