QUANTITATIVE STIMULATION OF NERVE 461 



tissue. The nerve, as usual, must be enclosed in a moist 

 chamber, a convenient form of which, as employed in practice, 

 will be seen in fig. 291. 



I shall next give a few records in illustration of the ease 

 and efficiency with which this mode of stimulus may be 

 applied. These records will show the characteristic varia- 

 tions of response given by the nerve under different con- 

 ditions. When making records of electrical responses with 

 frog's nerve, under electrical stimulus, Dr. Waller obtained 

 responses of three different types. The first of these was 

 the normal, and consisted of negative responses ; the second 

 was diphasic ; and the third was the abnormal positive. This 

 last he regarded as characteristic of stale nerve. 



These normal negative responses of the first of the three 

 classes were found by him to undergo enhancement after a 

 period of tetanisation ; while the third, that of the abnormal 

 response of stale nerve, underwent a change into diphasic, 

 or a reversal to normal, after tetanisation. 



From the fact that carbonic acid enhances the normal 

 negative response of nerve, Dr. Waller has suggested that 

 the enhancement of response in normal nerve after tetanisa- 

 tion, and the tendency of the modified nerve to revert to the 

 normal, are results of the hypothetical evolution ot carbonic 

 acid in the nervous substance, due to metabolism accompany- 

 ing excitatory reactions. It must be said, however, that no 

 trace of the presence of carbonic acid has yet been detected 

 in such cases. I shall be able to show, moreover, that these 

 effects are in no way due to the evolutions of carbonic acid, 

 but take place in consequence of molecular changes induced 

 in the responding tissue, which find concomitant expression 

 in changes of conductivity and excitability. 



I shall now give records of responses of these various 

 types obtained under the action of thermal stimulus. In 

 order to exhibit the effect of tetanisation I give, in fig. 275, 

 a series of normal responses by induced galvanometric 

 negativity, given by nerve of frog in its normal excitatory 

 condition. This nerve was then subjected to tetanic thermal 



