498 ELECTRO-PHYSIOLOGY CHAP. 



diminution during the experiment. It is not improbable that 

 this is due to a (positive) after-effect of excitation of the 

 nerve, produced by constriction. In such a case it is best 

 to let the variation decline, and then excite electrically. In this 

 way much more distinct positive variations are exhibited. 



The order of the deflections produced by these latter is almost 

 invariably such that after the expiry of the latent period and 

 eventually of the negative fore-swing, the positive variation is 

 rapidly initiated, and then becomes gradually slower, as if an 

 antagonistic effect were asserting itself; sometimes it continues 

 only for quite a short time, or even swings back a little, in the 

 sense of a negative variation finally, however, if the excitation 

 is prolonged, the positive effect breaks through again, and the 

 deflection becomes more characteristic. We are of opinion that 

 this retardation in the course of the positive variation is actually 

 to be referred to the opposite action of a simultaneously excited 

 negative variation, so that, as always, the visible deflection is 

 really the resultant of two antagonistic components. It is 

 determined by the preponderance of one or the other of these 

 forces. 



To this we must also refer the fact that, at a certain stage of 

 dehydration which is safe to appear when the frog is dried 

 simply by long detention in a dry chamber without water the 

 excitation of the sciatic, on leading off from the skin of the leg, 

 which usually gives a strong ingoing current only, generally 

 produces a distinct and fairly vigorous negative variation at the 

 first excitation, followed by a weaker positive effect. On repeating 

 the excitation at a later stage, no definite effect is usually 

 apparent ; but a slight swinging to and fro of the magnet, or 

 oscillation at its zero, indicates an interference of antagonistic 

 forces, which are nearly balanced. Under these conditions, a 

 complicated variation may result from tetanising, which consists 

 of four phases an initial negative deflection, soon interrupted by 

 an essentially stronger positive phase, which again swings back 

 in a negative variation, and finally the magnet is again slowly 

 reversed in the direction of a positive swing. The whole of this 

 complicated process occurs during the excitation. The best way 

 to observe the order of the separate phases is to take a frog at 

 the stage of dehydration in which the skin of the leg still gives 

 a marked ingoing current, and each excitation is accompanied 



