If a special mode of contraction ought to be designed for the muscles by a particular expression, such expression could be only derived from the property which they have of contracting from the influence of the will; but this property is foreign to their texture, and comes to them from the brain only; for as soon as they cease to communicate with this organ directly by means of the nerves, they cease also to be the agents of voluntary motion. These considerations lead us to examine the limits which are placed between the one and the other kind of contractility. We have seen that those which distinguish the two modes of sensibility, appear to be derived only from the greater or less proportion of this power; that in a certain proportion sensibility is of the animal kind, in a certain inferior proportion, of the organic kind, and that frequently from an augmentation, or diminution of intensity the two sensibilities reciprocally borrow their respective characters. We have seen a phenomenon almost analogous to this in the two subdivisions of the organic contractility. But this is not the case with regard to the two great divisions of contractility considered in general. The organic can never be transformed into the animal contractility. Whatever be its increase of energy, it constantly remains the same in its nature. The stomach, the intestines frequently assume a susceptibility of contraction, which makes them rise up and produces in them[34] the most violent motions by the most simple stimuli, but these movements preserve at all times their peculiar type, their primitive character; and have never been regulated by the brain. From whence proceeds this difference in the phenomena of sensibility and contractility? I cannot in a precise and rigorous manner resolve this question. VII. _Of the extensibility and contractility of texture._ I shall now proceed to examine the properties, which depend on texture only, on the organic arrangement of the fibres of the different parts. These are extensibility and contractility. They both succeed each other, and are connected in the same way, as in the vital phenomena, the organic and animal sensibilities are related to their respective contractilities.