326 COMPARATIVE ELECTRO-PHYSIOLOGY 



excitable tissue may be expected to show earlier or greater 

 fatigue, than, other things being equal, a less excitable 

 tissue. 1 Thus, under strong or long-continued stimulation, 

 the excitability of the originally more excitable B may be 

 depressed, so as to fall below that of A, with consequent 

 reversal of response. This will be seen clearly in a typical 

 experiment on the pulvinus of Mimosa. Electrical con- 

 nections were here made with the upper and less excitable 

 surface A and the more excitable lower surface B, records 

 being then taken of normal responses to equi-alternating 



Ki(i. 198. Photographic Record showing Reversal of Normal Response 

 in Pulvinus of Mimosa due to Fatigue 



(a) Series of normal responses, direction of current being from more 

 excitable lower to less excitable upper ; (b) Reversed responses in 

 same specimen, due to previous tetanisation, causing fatigue. 



electric shocks. The stimulus employed was of moderate 

 intensity, the secondary being placed slightly overlapping 

 the primary. The responses (fig. 198, a) are seen to be 

 normal, the responsive current being from the lower to 

 the upper. They also show signs of slight fatigue, their 

 amplitude undergoing diminution. The secondary was then 

 pushed over the primary, and the tissue subjected to the 

 consequent intense stimulus for two minutes continuously. 



1 In this matter, the nature of the tissue must be taken into consideration, 

 nerve, for example, being less subject to fatigue than muscle. 



