422 COMPARATIVE ELECTRO-PHYSIOLOGY 



chapter to exhibit normal excitatory contraction, and it will 

 be by the study of these motile responses in nerves, and their 

 variations under different condition, that we shall be able 

 unerringly to relate the abnormal responses sometimes seen 

 in the retina to those changes of condition to which they are 

 due. 



With regard to these abnormal responses, I have already 

 shown that various tissues exhibit them under the two 

 different conditions of sub-tonicity and fatigue. With regard 

 to nervous tissue in particular, however, I may refer here, 

 by anticipation, to results which will be given in detail 

 in Chapter XXXV. concerning the mechanical response 

 of nerve and its variations. In normal conditions of ex- 

 citability the nerve gives response by contraction, and this 

 is its true excitatory response, concomitant to the electrical 

 response of galvanometric negativity. This excitatory re- 

 sponse, then, whether by contraction or by negativity, will 

 here, as in preceding chapters, be designated ' negative.' 

 We have seen that the maintenance of the normal condition 

 of a highly excitable tissue depends on its supply of energy. 

 Hence, when such a tissue is isolated from the organism of 

 which it forms a part, it is liable to fall below its normal 

 tonic level. This depressed condition, however, does not 

 connote any permanent chemical depreciation, but only a 

 temporary depression of its fund of energy. Under such 

 an induced lowering of the tonic condition, the response is 

 reversed to positive. But under continuous stimulation, 

 excitability is again enhanced, and the abnormal positive is 

 converted to normal negative. 



Thus we obtain, in a somewhat sub-tonic tissue, the 

 following results : 



(1) Under a short-lived or instantaneous stimulus, whose 

 effective value falls below the true excitatory level, response 

 is positive (expansion). 



(2) Under the continuous action of stimulus the effective 

 value is at first below, but after a time rises above, excitatory 

 efficiency. In consequence of this we obtain a first phase of 



