310 PHYSIOLOGY OF MUSCLES AND NErxVES. 



closing is near the katbode, on opening is near tho anode. 

 This gives rise to the question, whether it is possible to 

 expose the nerve to the action of one electrode alone. This 

 may be done, in the case of men or other animals, by placing 

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

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

 closing pukations 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 may be called unipolar, 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 

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

 ever, are physiologically of no special interest. 



9. Tangent GalvanoiMeter (p. 162). 



In the ordinary tangent-galvanometer a small magnetic 

 needle is placed in the centre of a, comparatively, very large 

 circle, thi'ough the periphery of which the current is made to 

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

 does not alter essentiall}^ as regards the current, the action of 

 which may therefore be regarded as directly proportionate to 

 its strength ; and from the oj^posed action of the current, 

 and. of the force of attraction which the earth exercises on 

 the needle, which must also be regarded as constant, it is 

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

 trigonometric tangent of the angle of deflection is propor- 

 tionate to the strength of the current. 



Such tangent-galvanometers are, however, only adapted 

 for measuring powerful currents. The galvanometer which 

 we have described, adapted for very Aveak currents, is differ- 

 ent. But if, as was assumed, all the deflections which are 

 to be measured are but very small, we may still assume tliat 

 the mode of the influence of the current on the magnet is 



