QUANTITATIVE STIMULATION OF NERVE 465 



duction of excitation, and the enhanced excitability of the 

 responding-point, give rise to an increased amplitude of 

 response after tetanisation, as already seen in fig. 275. In a 

 depressed nerve, as the transmitted effect is positive, and the 

 tendency of the responding point itself, owing to sub-tonicity, 

 is to the abnormal positive, the record will exhibit the 

 abnormal positive alone, as in fig. 276. But under a series 

 of successive stimuli, the conductivity and excitability of the 

 tissue are both gradually raised, and the effect of this is seen 

 in the consequent gradual restoration of the normal negative 

 response, through the intermediate diphasic (fig. 277). Or, if 

 we do not wish to trace out the intermediate steps of transi- 

 tion, we may tetanise the depressed nerve for a certain 

 length of time, and record only the terminal change to 

 the restored normal negative, as is seen in fig. 276. 



Taking one of the extreme cases say that in which the 

 response to transmitted stimulus is positive, and is converted 

 into normal negative after tetanisation we see that the first 

 result is due to inefficient conductivity, allowing only the 

 hydro-positive effect to cause response. After this, increasing 

 conductivity, making an increasing transmission of true exci- 

 tation possible, gives rise to a diphasic, and ultimately to 

 the normal negative response. This result is analogous to 

 the three types of responses positive, diphasic, and negative 

 which we have already obtained with the imperfectly con- 

 ducting tissue of the petiole of cauliflower and the tuber of 

 potato (figs. 47, 48). We there saw that where excitatory 

 efficiency of transmitted stimulus was sufficiently great, it 

 gave rise to the normal negative response. When this, how- 

 ever, was not so great, we obtained the diphasic. Finally, 

 when the true excitatory effect could not be transmitted, 

 only the abnormal positive response appeared. That 

 gradation by which the transmitted stimulus was made 

 fully, partially, or non-effective, to induce true excitation, 

 was simply and most conclusively carried out in the case of 

 the potato, by removing the point of stimulation to an 

 increasing distance from the responding point. In the cases 



H H 



