THE EXCITATION OF NERVE FIBRES 



263 



In this way it has been found that the energy of a minimal effective stimulus for frog's 

 nerve is about 1Tr Vo of an er g- 



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 ex- 

 penditure of energy, which he calls the "characteristic" of 

 the tissue in question. To this point we shall have occasion 

 to refer later. 



8 



FIG. 110. Arrangement of apparatus 

 for the excitation of a nerve by 

 means of condenser discharges. 



B, battery ; R, rheochord ; c, rider 

 of rheochord ; s, switch (Pohl's re- 

 verser without cross wires) ; c, con- 

 denser ; n, nerve ; m, muscle ; e, non- 

 polarisable electrodes. 



When using the make and break of a constant 

 current as a stimulus, the first fact of importance is 

 the relation of the seat of excitation to the poles 

 by which the current is led into or out of the ex- 

 citable 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. Ill), 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 con- 

 stant 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 elec- 

 trode by which the current leaves the 

 muscle rises before the other lever. On the other hand, on breaking the 

 current, the lever at the anode rises first, showing that the anodic half of 

 the muscle contracts before the cathodic half. 



(b) The irritability of a muscle, i.e. its power of responding to a stimulus 

 by contracting, is intimately dependent on the life of the muscle. If the 

 muscle be injured or killed at any spot, its irritability at this spot will be 

 therefore diminished or destroyed. Hence, if we stimulate a muscle at the 

 injured spot, no contraction will ensue. This fact may be used to demon- 



FIG. 



111. Sartorius clamped in middle and 

 attached to levers at either end. 



