THE REFLEX RESPONSE OF THE ORGAN. 5 8q 



In the case of both Torpedo and Gymnotus, there is apparent evid- 

 ence that a succession of excitatory states leaves the electrical nerve cells 

 at a rate which may be at least 100 per second. It would be rash to 

 assume that this rate of outflow occurred from each central electrical 

 nerve cell, for in both these fish there are a large number of such cells. 

 But in the case of Torpedo the nerve cells are all definitely localised in 

 a portion of the medulla oblongata, and thus form as a whole the efferent 

 cells of one nerve centre ; the whole centre may possibly discharge at this 

 rate. It is, however, not clear in either Torpedo or Gymnotus what share 

 the self-excitation of the organ takes in the production of the rapid 

 succession of responses. Until the organ discharge has been recorded 

 by means of such an instrument as the capillary electrometer, the 

 responses evoked by fresh central nervous impulses cannot be separated 

 from those of the organ rhythm. At present, therefore, any such rapid 

 rate of central outflow of nervous impulses is not proved, and is on 

 a priori grounds improbable. 



The most interesting case is undoubtedly that of the Malapterurus, 

 for, as each organ is innervated by the terminal branches of one axis 

 cylinder, the character of the reflex response in this fish must indicate 

 that of the efferent output of the single nerve cell from which the 

 axis cylinder springs. The reflex response of the organ of Malapterurus 

 has been studied in the living specimen by the aid of the capillary 

 electrometer, and the following characters have been thus determined. 1 

 When the skin of the fish is excited, either mechanically or electrically, 

 a reflex response is evoked after a comparatively long period of delay. 

 This delay may be as little as 0023 sec, or as much as - 5 sec. The 

 smaller number, after deduction of the transmission time along the 

 main nerve-paths, and of the delay in the organ itself, leaves a remainder 

 of 0'008 sec. for the central time. This duration, 0'008 sec, is thus the 

 time occupied by the transmission of the excitatory state through the 

 central arc. 



As regards the character of the response itself, a difficulty arises, due 

 to the capacity for self-excitation previously described ; from the absolute 

 regularity of the effects produced during this peripheral organ activity, it 

 is, however, easy to distinguish, in the photographic records of electro- 

 meter excursions, between the results of such auto-excitation and the 

 arrival at the organ of a new excitatory state along the efferent nerves. 



This distinction is further facilitated by the circumstance that, in 

 Malapterurus, whereas the separate responses due to auto-excitation occur 

 at rates of from 100 to 300 per second, those due to a fresh central 

 outflow are of a very different order. The shortest interval between 

 two separate responses of a reflex character yet observed was T V sec. 

 (0*085), and the average interval was as much as \ sec. (0 - 25). Even 

 this slow rate of central discharge could not be maintained, the maximum 

 number in any series being only five, and the average number two to 

 three. If the reflex organ response, when evoked by impulses reaching 

 it through its efferent nerves, gives a true picture of the central dis- 

 charge, then in the Malapterurus a second excitatory state cannot emerge 

 from the single cell at an interval of much less than J-g- second, whilst 

 the cell itself, after two or three such displays of activity, is fatigued. 

 The same susceptibility to central fatigue occurs in both Gymnotus and 

 Torpedo, and hence it is not improbable that the so-called immunity of 



1 Gotch and Burch, loc. cit. 



