Each moveable point of the animal frame is always between two opposite muscular forces, between those of flexion and extension, of elevation and depression, of adduction and abduction, of rotation without and rotation within, &c. This opposition is a condition essential to the motions; for in order to perform one of them, it is necessary that the moveable point should be in the opposite motion; in order to bend, it is necessary that it should be first extended, and reciprocally. The two opposite positions which a moveable part takes, are for it alternately the point of departure and the point of arrival; the two extremes of these positions are the two limits between which it can move. Now between these limits there is a middle point; it is the point of rest of the moveable part; when it is found there, the muscles are in their natural state; when it passes it, some are extended, others are contracted, and such is their arrangement, that the contraction and extension which take place in an opposite direction, are exactly in direct ratio. Hence, in the reciprocal influence that the muscles exert upon each other, they are alternately active and passive, power and resistance, organs moved and organs which move. The effect of every muscle which contracts is not then only to act upon the bone in which it is inserted, but also upon the opposite muscle. Between two muscles thus opposed, there is often no solid intermediate organs, as in the lips, the linea alba, &c. The muscle of one side acts then directly upon that which corresponds to it, in order to distend it. Now this action of the muscles upon each other is precisely the phenomenon of the antagonists; two muscles are such when one cannot contract without elongating the other and vice versa. Let us examine in this phenomenon the part of the contractility of texture; it is necessary to distinguish its influence from that of the vital forces, which has not been heretofore sufficiently done.