POLAR EXCITATION OF NERVE 91 



break from the anode, and the same is found to be the case with 

 nerve. As in the analogous experiment upon muscle, the starting 

 points of the two excitations have been determined by time measure- 

 ments of the two latent periods of the make and break contractions. 

 The electrodes are separated as widely as possible ; then if the stimulus 

 start from the upper electrode, the latent period should be longer 

 than if it started from the lower. Experimenting in this way it is 

 found that if the current be ascending, the latent period of the contrac- 

 tion on make is longer than that of the contraction on break. From 

 this we infer that the contraction on make starts from the upper elec- 

 trode, i.e. the kathode, and that on break from the lower, i.e. the anode. 

 A converse result is found for a descending current. 



We may also state the fact of polar excitation in the following 

 way. The production of katelectrotonus and the disappearance of 

 anelectrotonus stimulate a nerve. Or, again, the passage of a nerve 

 from a condition of lesser to one of greater excitability stimulates it. 



In examining the effect of opening and closing a constant current 

 through a nerve the results obtained are found to vary with the 

 strength and direction of the current, and to study these variations we 

 must possess some means of conveniently varying the strength of the 

 constant current. This is afforded by either of the two following 

 pieces of apparatus : — ■ 



The monochord is an application of the principle of the deriving circuit and 

 is employed for varying the strength of current to be sent through a nerve or 

 other tissue. Fig. 77 illustrates the method of using a monochord. A 

 current is sent through a stretched wire, a b, which must not be of too 



low resistance. The two electrodes are E 



connected, one to one terminal, a, of 

 the wire, the other to a movable con- 

 tact, s. Supposing now the electrodes 

 e to be bridged by a nerve, when the key 

 k is closed the current on reaching a 

 divides, part passing along a s, the other 

 through the nerve. The amount of 

 current passing through the nerve is 

 inversely proportional to the resistance FlG . 77._To Illustrate the Prin- 

 of that part of the circuit, and directly ciple of the Monochord. 



proportional to the potential difference 



between a and s. If s be brought nearer to a this potential difference is 

 decreased in direct proportion to the distances between A and s in the two 

 positions. Where, as in the case we are considering, the resistance in the 

 nerve circuit is very high, and therefore the resistance of a s may, in com- 

 parison, be neglected, the current through the nerve is directly proportional 

 to the potential difference only, i.e. is measured by the length A s. 



If we wished to know exactly the value of this current, it is necessary to 

 measure the resistances of the nerve circuit and of a r, and from these the 

 total amount of current can be calculated. For most purposes, however, it is 

 sufficient to state the current strength as measured by the lengths between a 

 and s in different positions. 



