SUMMARY. 227 



negative on the, cross-section. The greatest positive 

 tension is in the centre of the longitudinal section. 

 During activity the differences in tension decrease. 



(5) The electric plate of electric fish is, when qui- 

 escent, electrically inactive ; influenced by the nerves, 

 the one side becomes electrically positive, the other 

 negative. 



(6) In the glands the base is positive, the opening 

 or inner surface negative ; during activity the dif- 

 ferences in tension decrease. 



These propositions state only the most important of 

 the conditions which have been shown by experiment. 

 On the outer surfaces of the tissues examined we found 

 differences in electric tension ; and we found reason 

 to believe that the causes of these differences in tension, 

 which occur with great regularity, must be situated 

 within the tissues themselves. We now have to dis- 

 cover these causes, and this is not so easy to do as 

 it perhaps appears at first sight. Difficult as it may be 

 to calculate the tensions which must prevail at each 

 point on the outer surface of a given body, within 

 which an electromotive force is situated, yet the diffi- 

 culties in this case may be overcome by skill. It is 

 different, however, when the problem is reversed, when, 

 the distribution of the tension having been experi- 

 mentally found, it is required to discover the seat of 

 the electromotive force. The difficulty in this case 

 consists in the fact that the task is undefined, and 

 that many very various solutions may be found. More- 

 over the task is rendered yet more difficult by the 

 fact that we do not know whether one or many elec- 

 tromotive forces are present, situated in different parts 

 of the body. 



