ANIMAL AND VEGETABLE 79 



inductive capacity was uniform throughout, and this, 

 obviously, is not the case. 



Retardation, or the portion of the current retained upon 

 the surface of the wire, is also dependent upon, among 

 other things, the length and diameter of the wire or, in 

 other words, upon its resistance. And here note should 

 be taken of the fact that the effect of capacity is to produce 

 prolongation at the end as well as retardation at the com- 

 mencement of a current ; so that a current takes longer to 

 leave the line than it did to enter it. 



" In nerves," I learn from Landois and Stirling, " the 

 resistance is two and a half million times greater than in 

 mercury, while in animal tissues it is almost a million times 

 greater than in metals." Taking the specific resistance of 

 copper as 1, mercury (at 57) is approximately 50, so that 

 the resistance of the nerve, taken longitudinally, would be 

 50,000 times greater than that of copper. For liquids the 

 resistances are enormous as compared with metals, and 

 they are subject to chemical decomposition or change in 

 the process of conduction. 



It is, of course, extremely difficult, if not impossible, to 

 calculate accurately the resistance of a living nerve 

 relatively with that of a copper wire unless we are given the 

 exact sectional area of the nerve-conductors, and, pro- 

 bably, not even then. But for curiosity's sake it may be 

 well to see how the 50,000 times increase of resistance 

 works out. j, 



We will take two round pure copper wires of sectional 

 areas of 0-01 and 0-02 in. respectively, and suppose them 

 to be two nerves of the same diameter. 



The resistance of a copper wire of 0-01 in. corrected to 

 100 F. is 0-3677 ohm per metre, and if we, for convenience 

 of calculation, take the maximum length of a nerve to be 

 2 metres, we have 0-3677 X 2 x 50,000 = 36,770 ohms 

 as its total resistance, or 4- 6-5 = 5,657 ohms per ft. length 



