CONDITIONS AFFE CTING EX C IT A TOR Y RESPONSE. 5 7 3 



(a) The number of structural elements. — This is best illustrated in the 

 case of the torpedo organ preparation. The response is the summed 

 effect of changes of like sign, occurring simultaneously in each of a 

 superimposed series of electrical discs ; the number of such discs thrown 

 into simultaneous activity must therefore be a potent factor in deter- 

 mining the magnitude of the total effect. A simple experimental proof 

 is the comparison of the galvanometric effects produced when a portion 

 of the columns and the whole of the columns respectively are traversed 

 by the same exciting current. The exciting current which is thus 

 allowed to traverse the tissue and excite the organ directly, is prevented, 

 by the use of an appropriate rheotome, from entering the galvanometer 

 circuit, which is coupled up with the preparation about '005 sees, after 

 the exciting induced current has been passed through the organ strip. 

 If the portion of organ traversed by the exciting current is small, then 

 the response is small ; as it increases in length, so as to include greater 

 lengths of column, the response shows a proportionate increase in 

 intensity. 



A further consideration is suggested by this simple experiment. 

 The response is an effect, localised in each transverse disc or plate with 

 its innumerable nerve-endings. There can be no propagation of the 

 excitatory change from one plate to another, for the simple reason that 

 each plate is structurally complete in itself, and is not connected by its 

 differentiated protoplasm with its neighbours. Simultaneous activity 

 is ensured by simultaneous excitation of the nerves to all the plates of 

 the series, and the response of the organ thus differs from that of either 

 a muscle or nerve fibre, since in these the change at the point first 

 aroused is successively assumed by neighbouring portions of the proto- 

 plasmic continuum, which have not been the seat of the initial excitatory 

 disturbance. 



(b) The size of the structural elements. — The intensity of the response 

 increases as the plates increase in size ; and since the size is greater 

 the larger the fish, it is always more pronounced in fully grown than it 

 is in young fishes. The increase is too pronounced to be accounted for 

 by the diminution in the electrical resistance offered by the larger 

 organs, for such diminution is small compared with the greatly aug- 

 mented intensity of the organ response. Further, the capillary electro- 

 meter shows a similar increase, hence it is due to the greater capacity 

 for electromotive change possessed by each plate of the adult organ. 



(c) The intensity of the stimulus. — As in all excitable tissues, so in 

 the electrical organ preparation, a stimulus must be of a certain 

 intensity before it can evoke an appreciable response. When this 

 intensity is reached, the response can be further increased in magni- 

 tude by augmenting the stimulation. The minimal exciting intensity 

 of electrical induced currents is much greater than that adequate to 

 evoke a response in a muscle preparation of the frog, whether this 

 is aroused by indirect or direct stimulation. In the case of the 

 Malaptcrurus organ, the ratio between the amounts of such a minimal 

 stimulus as the break-induced current applied, on the one hand to the 

 nerve of the organ, and on the other to the nerve of the gastrocnemius 

 of the frog, may be as much as 100 to 1. 



It has been already pointed out that the threshold of exciting 

 efficiency is so high in the case of chemical stimuli, that such reagents 

 as NaCl, etc., evoke no response in the organ preparation, even when 



