THE EXCITATION OF NERVE FIBRES 



301 



muscle and contractile tissues generally, and probably hold good for 

 all irritable living tissues. 



It is said that an anelectrotonus takes some time to attain its 

 full height, and a catelectrotomis reaches its maximum almost directly 

 after the current is made, and that it is on this account that a current 

 of very short duration excites only at the make, the break occurring 

 before the anelectrotonus is deve- 

 loped enough for its disappearance 

 to cause a stimulus. 



Other things being equal, a 

 current of given strength causes 

 a stronger excitation the greater 

 the length of nerve that it flows 

 through. It must be remem- 

 bered, however, that the nerve 

 offers considerable resistance to 

 the passage of the current, and 

 so, to keep the current constant 

 while increasing the length of 

 intrapolar nerve, we must largely 

 increase the electromotive force 

 employed. 



A very convenient method of show- 

 ing the effect of the length of intrapolar 



nerve on excitation has been suggested by Gotch. The two sciatic nerves of 

 a frog are dissected out, one of them being in connection with the gastroc- 

 nemius. These are first arranged as in Fig. 118. a, b, and c are three non- 

 polarisable electrodes, the terminals of a constant battery being connected to 

 a and c. The position of the rider on the rheochord is then ascertained at which 

 make of the current just excites contraction in the muscle of nerve 2, the current 

 in this case passing from a to b along nerve 1, and from b to c along a small 

 piece of nerve 2. We will suppose that eleven units of current are necessary 

 to produce excitation, b is then withdrawn and the nerve 2 laid on a (Fig. 118, B), 

 so that the current can now pass from a to c entirely through a long stretch 

 of nerve 2. On again seeking the minimal stimulus, it will be found that a 

 smaller current is sufficient to excite, contraction being obtained with seven 

 units. Since the length of nerve traversed, and therefore the resistance to the 

 current, are the same in both cases, it is evident that a current is more effective 

 the greater the length of excited nerve that it traverses. 



A nerve cannot be excited by currents passed transversely across 

 it, since in such cases the anode and kathode lie so close to one another 

 in a nerve-fibril, as it is traversed by a current, that their effects 

 counteract one another. 



FIG. 118. 



