310 riivsiol.ut.v OF MUSCLES AND NKUVI 



closing is near the kathode, on opening is near tli anode. 

 'I'liis Drives ri.se to the question, whether it is possible to 

 expos*' the nerve to the action of one electrode :iluiie. This 

 may lie done, in the case of men or other animal-. by placing 

 one electrode on tlie nerve, the other on a remote part of 

 the body. If the kathode is situated on the nerve, only 

 closing pul.-at ions are obtained; if the anode is situated on 

 the nerve, opening pulsations are alone observed. If the 

 currents are very powerful, excitement may certainly occur 

 at the point where the nerve meets the adjacent tissues. 

 This form of nerve irritation maybe called ii,ii/i['tr, though 

 in a different sense from that in which the name is usually 

 used in cases where only one wire is laid on the nerve, and 

 vet currents may flow through the nerve. Such cases, how- 

 .-. are physiologically of no speci.-il interest. 



9. T\M:::.NT CALVANO.MI;TI:I; (p. Kii'). 



In the ordinary tangent-galvanometer a small magnetic 

 needle is placed in the centre of a, comparatively, very lar-e 

 circle, through the periphery of which the current is ma le to 

 pass. When the needle is deflected, the position of its poles 

 does not alter essentially us regards the cm-lent, the action of 

 which may therefore I e re-aided as directly proportionate to 

 its strength: and from the opposed action of the current, 

 and of the force of attraction which the earth c\erci.-es on 

 the needle, \\hich must also be regarded as constant, it is 

 evident that the two forces must be in equilibrium, if the 

 trigonometric tangent of the aii^le of deflection is propor- 

 tional, i to 1 lie si ren^lli of the current. 



Such tangent-galvanometers are. however, only adapted 

 f -r measuring powerful currents. The ^ih aiiom. t r which 

 we have described, adapted for very \\eak currents, i- dill'er- 

 enl. Hut if, as wa> a ..-limed, all the deilrrtinns which are 

 to 1 e measured are Iml very small, we may still assume that 

 the mode of the influence ,(' i| M . current on the magnet is 



