NATURAL HISTORY OF ORGANIZED BODIES. 303 



muscle, to a study of the action of tlie nerves wliicli proceed to the electric 

 apparatus. In incpiiring wlietlier these nerves are similar to the motor-nerves, 

 he has, in effect, found the reseml)lance perfect: 1. That the section of the for- 

 mer suppresses the spontaneous discharges of the animal, just as the section of 

 the motor-nerves suppresses voluntary movement in the mnscles to which they 

 are distributed. 2. That the excitation of the periplieral end of an electric nerve 

 provokes a discharge of the apparatus, as the excitation of a motor-nerve jjro- 

 vokes a shock of the corresponding muscle. 3. That the excitation of the cen- 

 tral end of the electric nerve provokes in the animal no phenomenon of sensi- 

 bility, as none is occasioned when the central end of a nerve of movement is 

 excited. 4. M. Moreau having poisoned a tttrpedo with strychnine, which com- 

 municates to the motor-nerves a series of re])eated excitations and throws the 

 nmscles into tetanic convulsion, found that this drug jjrovokcd in the electric 

 apparatus very frequent discharges, similar in all respects to the convulsions of 

 a tetanized muscle. 



The phenomena of scnsibiVittj are, within certain limits, subjected to the same 

 laws with the phenomena of movement. We verify with regard to both the 

 law which teaches us that activity exhausts the function, and that repose restores 

 it. A lively sensation fatigues the sensibility, exhausts or abolishes it for a 

 certain time, while by repose its previous intensity is renewed. 



Let us take as an example the most complex, but at the same time most inter- 

 esting of our sensitive manifestations, the sight. When we look at a very bright 

 luminous object, the point of our retina on which its image falls is vividly 

 excited; it becomes fatigued, and if we turn the eyes on a field of a uniform clear 

 color, we see on it a darker spot, presenting the exact form of the bright object 

 by which our vision had been impressed. This spot is owing to the fact that the 

 fatigued poipt of our retina no longer perceives the luminous sensations with the 

 customary intensity. The more In'illiant the body observed, and the longer the 

 time we have observed it, so much darker and more persistent is the ensuing 

 image. Repose of the sight causes this subjective image, as it is called, gradu- 

 ally to disappear. 



The i'atigue of our retina may be restricted to certain elements of sensation, 

 if we have received the impression of only certain elements of the light. Thus, 

 our vision ma}' be fatigued for the blue, the red, or the yellew sei)arately. Sup- 

 l)Ose, for example, that a red wafer be placed on a sheet of A\hitc paper, and 

 that we look upon it intently for some instants. Let us now remove the wafer 

 without ceasing to look at the same point ; we shall immediately see a grccti disk 

 of the same dimensions with the wafer appear in its place. The reason thereof 

 is: that in the white light of the paper our eye cannot perceive so vividly the 

 red rays in the point of the retina which is fatigued with that color, and as all 

 the other rays are there perceived, these form by their fusion the complementary 

 color of red, namely, green. In the same way, a green wafer would leave after 

 its disa|>pearance a red sul)jective image ; a yellow wafer would give a violet 

 image, &c. 



I shall not dwell longer on examples of the very general law that every func- 

 tion tvltich is exerted is momentardij ex]iaustcd,cutd tlud if is restored hy repose. 

 Let us proceed to a brief consideration of laws of another order in the jihenomena 

 of life. We will take, for example, the influence of functions upon one another. 

 On this subject I may be allowed to adduce certain general views which 

 appear to me to result from the observation of phenomena and from physiological 

 experiment. A law of harmony among the functions of life will, I think, be 

 admitted W'ithout dithculty j that is to say, that if one function reacts on another 

 it influences the latter in such manner as to derive therefrom advantage for itself. 

 To develope this idea, I will present a few examples : It has been already said 

 that any nmscular action has need of being maintained by the circulation of the 

 blood; now the action favors this circulation and renders it nuire rapid. 



