THE EXCITATION OF NERVE FIBRES 



295 



and the electromotive force by which it is charged, we can estimate the energy 

 of the charge sent through the nerve. 



E (energy in ergs)* = 5FV 2 

 (F = capacity in microfarads ; V = electromotive force in volts). 



In this way it has been found that the energy of a minimal effective stimulus 

 for frog's nervs is about y^j^ of an erg. 



The amount of energy necessary to excite the nerve will vary with the rate 

 at which the condenser is allowed to discharge through the nerve. Its rate 

 can be modified by altering the resistance in the discharging circuit or by 

 altering the electromotive force of the charge. This 

 method has been adopted by Waller in determining 

 the rate of change at which excitation is obtained 

 with a minimal expenditure of energy, which he calls 

 the "characteristic" of the tissue in question. To 

 this point we shall have occasion to refer later. 



When using the make and break of a con- 

 stant current as a stimulus, the first fact of 

 importance is the relation of the seat of exci- 

 tation to the poles by which the current is 

 led into or out of the excitable tissue. 

 We have already seen that when a 

 current is passed through a muscle 

 or nerve the muscle contracts only 

 at make or at break of the current, 

 no propagated excitatory effect being 

 produced during the passage of the 

 current. The excitation at make is 

 obtained with a smaller current than 

 the excitation at break. 



Besides this difference in intensity, there is a difference in the 

 point from which excitation starts. A make contraction starts from 

 the cathode, a break contraction from the anode. This is well shown by 

 the two following experiments : 



(a) A curarised sartorius muscle of the frog (Fig. 112), with its 

 bony insertions still attached, is fastened at the two ends to two 

 electrodes, which are able to swing when the muscle contracts, and are 

 attached by threads to levers which serve to record the contraction . 

 The middle of the muscle is then fixed by clamping^it lightly. A 

 circuit is arranged so that a constant current can be sent through the 

 electrodes and the whole length of the muscle. It is found, on making 

 the current, that the lever attached to the cathode that is, to the 



* An erg is the amount of work produced or energy expended by the action 

 of one dyne through one centimetre. 



A dyne is the force which will give to a mass of one gram an acceleration of 

 one centimetre per second per second. 



FIG. 111. Arrangement of apparatus 

 for the excitation of a nerve by 



means of condenser discharges. 

 B, battery ; B, rheochord ; c, rider 

 of rheochord ; s, switch (Pohl's 

 reverser without cross wires); 

 c, condenser; n, nerve; TO, muscle; 

 e, non-polarisable electrodes. 



