NATURAL PHILOSOPHY. 133 



this quality of tension. Generally speaking, a \veak current produces feeble 

 contractions of the muscles, and slight effects upon the organs of sensation; 

 whereas a powerful current produces strong contractions and violent sensa- 

 tions. Both sensory and motive phenomena may be occasioned by the ap- 

 plication of any one of these currents, but their variations in intensity render 

 some more useful for one class of effects, and others for a second class. 

 Thus, Duchcnne has drawn considerable attention to the fact, that the ' extra 

 current 'acts very readily on the muscles, and that the ' induced current' 

 affects more powerfully than the extra current, the skin, nerves, and retina. 

 This difference of action he refers to a special elective power on the part of 

 the two currents respectively; but Becquerel has proved that, in reality, it is 

 merely dependent upon the difference of their intensity, the induced current 

 having much greater tension than the exti-a current. M. Becquerel has 

 shown, by a simple experiment, in which he modifies the arrangement of the 

 wires, that the effects which Duchenne attributes to the one current may be 

 obtained from the other, and vice versa. 



" In proportion to the intensity of the current employed, electricity has the 

 power of evoking the ordinary physiologic action of a nerve or muscle; of 

 occasioning excessive and perverted action; of exhausting the functional 

 activity for a time, or of destroying it altogether. In the first degree there is 

 sensation or motion, each of these being within the limits of physiologic 

 function; thus luminous appearances, gentle sounds, gustatory effects, etc., 

 on the one hand, and slight muscular contraction on the other, contraction 

 so slight as merely to exhibit the pei'sistcncc of muscular contractility, and 

 not to test its power, arc the results of applying an electric current of low 

 intensity. If a stronger current is employed, the impressions upon the sensory 

 organs become excessive in degree and painful in character; while, in the 

 place of gentle muscular contraction, there is distressing cramp or arrested 

 (inhibited) action in certain organs. A still more violent current exhausts 

 both nerve and muscle; and here sensation and contraction, though for a 

 time withdrawn, are capable of being restored by repose, or by the inverted 

 current; whereas the electricity may be so powerful as at once to put an end 

 to the vitality of the tissues i. e., to kill the nerve, limb, or individual 

 through which it passes. 



" It is owing to these different effects of variations in intensity that elec- 

 tricity may be employed both physiologically and therapeutic-ally for so 

 many different purposes. As a test of irritability, or a gentle stimulus of 

 weakened sensibility and contractility, the current of low intensity may be 

 employed. For the sake of displaying the inhibiting influences of the vagi 

 and the splanchnic nerves, or for awakening the torpid nervous centres of 

 an individual poisoned by opium or alcohol, a more powerful current is re- 

 quired. Whereas, for the relief of excessive muscular contraction, or of 

 neuralgia, a still more intense current, one that shall temporarily exhaust 

 the nervous function, may be employed. 



"Besides the quantity and tension of a current, the mode of its transmis- 

 sion exerts a notable influence upon its physiologic effects. Under this 

 head we place the different actions of the continuous and interrupted cur- 

 rents; and with regard to the former, the changes produced by altering 

 their direction ; and with regard to the latter, their convection by means of 

 moist or dry conductors, the rapidity or slowness of their interruption, and 

 the degree of pressure with which the conductors are applied. 



"The most general differences between the effects of the continuous and 



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