1899.] on the Electric Fish of the Nile. 115 



nerve was then brought forward. The shock even when evoked by a 

 single stimulus was shown to be rarely if ever a single one. Each 

 effect consists of a rhythmical series of electrical changes occurring 

 one after another in a perfectly regular manner at intervals of r lJ' to 

 3^0", the rate depending upon the temperature. By special experiments 

 it was shown that this rhythmical series is due to self excitation, 

 each change producing an electrical current of sufficient intensity to 

 excite the nerves of the tissue in which it was generated. It follows 

 that only the initial member of the series need be evoked by nervous 

 impulses descending the nerves, since the others must then ensue. 

 The potency of the organ as a weapon to be wielded by the fish is 

 thus enormously increased by its resemblance to a self-loading and 

 self-discharging automatic gun. The total electromotive-force of the 

 whole organ in a fish only eight inches long can reach the surprising 

 maximum of 200 volts, at any rate in the case of the initial shock. 

 The attainment of this maximum is due to the simultaneous develop- 

 ment of perfectly similar electromotive changes in each of the two 

 million discs of which the organ is composed. In a single disc the 

 maximal electromotive-force only amounts to from '04 to "05 volt, 

 and since in a small nerve an electrical change of '03 to *04 volt 

 has been observed, the large total effect is not due to any extra- 

 ordinarily intense electrical disturbance in each tissue element, but 

 to the tissue elements being so arranged that the effect in one 

 augments those simultaneously produced in its neighbours. 



Finally, the remarkable characters of the nervous connections of 

 the organ were described. Each lateral half of the organ, although 

 it has a million plates receiving nerve branches, is innervated by 

 one single nerve fibre and this is the offshoot of a single giant nerve- 

 cell situated at the cephalic end of the spinal cord. The structure 

 of this nerve-cell was displayed by means of microscopic sections 

 and by wax models made by G. Mann, of Oxford. As regards the 

 nervous impulses which the fish can discharge through this nerve- 

 cell, experimental results were described which show that the fish 

 is incapable of sending a second nervous impulse after a preceding 

 one until a period of T ^ second has elapsed, and that this interval is 

 rapidly lengthened by fatigue to as much as several seconds. The 

 inability of the central nervous system to repeat the activity of the 

 organ obviously presents disadvantages to the use of the shock as a 

 weapon for attack or defence, but such disadvantage is more than 

 counterbalanced by the property of the organ alluded to in the 

 earlier part of the lecture, viz. that of self-excitation, since a whole 

 series of shocks continue to occur automatically in rapid succession 

 provided that an initial one has been started by the arrival at the 

 organ of a nervous impulse sent out from the central nerve-cell. 



[F. G.] 

 1 2 



