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the existence of man and of the animals which resemble him, obliged to 

 keep up multifarious intercourse with every thing that surrounds them ; 

 thus they enjoy a very superior kind of sensibility, by means of which the 

 irnpresssons which affect some of their organs are perceived, judged, and 

 compared. This mode of sensibility might be more properly called per- 

 ceptibility, or the faculty of accounting to one's self for the emotions which 

 are experienced. It requires a centre to which t e impressions may be 

 referred, and therefore it exists only in the animals which, like man, have 

 a brain or some organ in its stead ; so that the zoophytes and vegetables, 

 wanting that organ, are equally destitute of this faculty. The polypi, 

 and some plants, as the sensitive, perform nevertheless certain sponta- 

 neous motions, which seem to indicate the existence of volition, and con- 

 sequently of perceptibility. But these motions are the result of an im- 

 pression, which does not extend beyond the part in which it is experi- 

 enced, and in which sensibility and contractility are blended. 



The almost latent sensibility of certain parts of the body, cannot be ab- 

 solutely compared to that of vegetable; since those organs whose sensi- 

 bility is so imperfect, manifest in disease a percipient sensibility, which 

 shows itself by acute pain, and it is even sufficient to change the stimulus 

 to which they are accustomed, to determine the occurrence of that phe- 

 nomena. Thus the stomach, on the parietes of which, the food does not 

 in health produce any perceptible impression, becomes the seat of very 

 distinct sensation and of dreadful pain, when a small quantity of poison- 

 ous matter is introduced into it. In like manner, we are not conscious of 

 the impressions excited in the parietes of the bladder or rectum, by the 

 collection of urme^or foecal substances, except when their contents have 

 become sufficiently irritating by their presence, to excite in a certain de- 

 gree, those irritable and sentient cavities, and to transform their obscure, 

 into a very distinct sensibility. Is there not reason to suspect, that our 

 unconsciousness, in health, of the impressions made on our organs by the 

 fluids which they contain, depends on our being accustomed to the sensa- 

 tions which they incessantly excite? so that there remains but a confused 

 perception, which in time disappears, and may we not, under that point 

 of view, compare all these organs to those of sight, hearing, smelling, 

 tasting, and touching, that are no longer irritable by stimulants, to which 

 they have long been habituated ? 



Two systems of organs, very different in their uses and in their quali- 

 ties, enter into the composition of the human body; they are as two liv- 

 ing and united machines, the one, formed by the organs of sense, the 

 nerves, the brain, the muscles, and the bones, serves to maintain its con- 

 nexion with external objects: the other, destined to internal life, con- 

 sists in the digestive tube, and the organs of absorption, circulation, 

 respiration, and secretion. The organs of generation in the two sexes 

 form a separate class, which, as far as relates to the vital properties, par- 

 takes of the nature of the other two. 



By the organs of sense, and the nerves which form a communication 

 between these organs and the brain, we are enabled to perceive or to feel 

 the impressions made onus by external objects : the brain, the true seat 

 of that relative sensibility, which might very justly be termed perceptibility 

 or the perceptive power, (Pott) when excited by these impressions, is able 

 to irradiate into the muscles the principle of motion, and to induce the 

 exertion of their contractility. This property, which is under the direc- 

 tion of the will, manifests itself by the sudden shortening of a muscle, 



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