We have just described separately the stimulant and the organ stimulated; each being separate there is no effect upon the sensible organic contractility; from their union alone results the exercise of this property. What happens in this union? We know not. To wish to know it, would be to wish to know how one body attracts another, how an acid combines with an alkali, &c. In attraction, affinity and irritability, we can only trace the phenomena to the action of bodies upon each other. This action is the utmost limit of our researches. But that which ought not to escape us here is, that in this last property, the action is never immediate. There is always between the stimulus and the organ something intermediate which receives the irritation; this intermediate organ is a delicate membrane and continuous with that of the arteries for the heart; it is a mucous surface for the gastric viscera and the bladder. This intermediate organ is more susceptible of receiving excitement than the muscle itself. I have uniformly observed that by irritating the internal surface of the heart, its contractions are greater, than by laying its texture bare externally by removing its serous covering and afterwards stimulating it. The same is true with regard to the organic muscles of the abdomen. Is there between the intermediate organ excited and the organ which contracts, any nervous communications that transmit the impression? I think not, the cellular texture is sufficient. In fact the serous surfaces and the organic muscles have only this texture as a means of union. The life of the first is in no way connected with that of the others, since they often leave them as we shall see, and yet they can transmit excitement to them. The pericardium and the peritoneum, irritated in their portion corresponding with the organ that we wish to move, produce a contraction in it. This fact is known to all those who have made the least experiment; it is almost always in this way that we stimulate the heart, the stomach, the intestines, the bladder, &c. By carrying the stimulus over the serous surface but very lightly, and so as not to communicate the motion to the fleshy fibres, we obtain a result. Yet simple contact is not sufficient to transmit the irritation; for example, by leaving the external layer of the pericardium applied to the heart and afterwards irritating it, the organ remains immoveable. If we separate the peritoneum from above the bladder, so as to break all the cellular adhesions, and afterwards reapply and stimulate it, the same immobility is observed.