524 



COMPARATIVE ELECTRO-PHYSIOLOGY 



intervening period of tetanisation will markedly enhance the 

 negative response. 



We have now seen that, by the direct mode of investiga- 

 tion afforded in mechanical response, we are able to trace 

 out the causes which determine the three types of response 

 found in nerves. It has been shown that all these are 

 brought about by the varying tonic condition of the tissue. 

 From this it is easy to understand that the three types of 

 electromotive responses in nerve are also due to the same 

 cause. In the experimental method 

 there employed, the variations of con- 

 ductivity appropriate to the tonic 

 condition are superposed on parallel 

 modifications of the excitability. Thus 

 not only is the responsive change of 

 a sub-tonic responding point positive, 

 but the effect which is transmitted to it 

 through sub-tonic conducting tissues is 

 also positive ; after tetanisation, how- 

 ever, the tonic condition of the tissue is 

 raised. The power of transmitting 

 true excitation, previously in abeyance, 

 is now not only restored, but gradually 

 enhanced to a degree, depending within 

 limits, on the amount of tetanisation. 

 The excitability also undergoes a similar 

 transformation, from the abnormal 

 positive to normal negative, which latter 

 again becomes enhanced to a degree that depends, within 

 limits, on previous excitation. These effects, seen in electrical 

 response to transmitted stimulation applied at a distance, 

 I find repeated also in the mechanical response of nerve, 

 under similar circumstances. That is to say, an isolated 

 nerve, by the very fact of its being cut off from its normal 

 sources of energy in the body of the intact animal, is apt 

 to be rendered sub-tonic, and under these conditions no 

 true excitation is transmitted, and it is only when the tonic 



FIG. 322. Photographic 

 Record showing Stair- 

 case Effect in Me- 

 chanical Response of 

 Frog's Nerve 



