Extensibility is manifested in the animal muscular system under many circumstances. The different motions of our parts render this property evident. Such is in fact the arrangement of the muscular system, that one of its portions cannot contract, without the distension of another. The thigh being strongly bent, the semi-membranosus, the semi-tendinosus and the biceps are elongated. The arm being carried out, the great pectoral is extended, being raised, the great dorsal and the teres major are stretched. All the great flexions bring into action this property in the extensors; all the extensions render it evident in the flexors. A muscle which is stretched by its antagonist is in a state purely passive; it is as it were for a moment abandoned by its contractility, or rather it possesses it, without its being brought into action; it is made to obey the motion that is communicated to it. Observe that in these cases, the distension is confined to the fleshy portion, and the tendon has no connexion with it; it remains the same, whatever may be the distance, of the points of attachment, for these points are nearer or more remote in the different extensions to which the muscles are exposed; the longest muscles yield the easiest. The sartorius, the posterior muscles of the thigh, &c. exhibit this phenomenon in an evident manner; as their position is accommodated to it. In general all the muscles remarkable for their length are superficial, and go most commonly to two articulations, sometimes even to three or four, as in the limbs. Now the number of these articulations renders the space comprised between the two points of attachment susceptible of very great variations, which the great extensibility of these muscles allows. It may be understood from what has been said above, that it is to the length of the fleshy fibres and to the whole length of the muscle, that its degree of extensibility is to be referred. Those in which many aponeuroses are intermixed, and which derive in part from these membranes or from tendons their length, possess less of this property. Hence why, in the same motions, muscles of the same total length become more or less short, more or less elongated in their fleshy portion. Observe however that when on the one hand the tendinous portion predominates much, and on the other that it is very delicate, it yields a little, as we see in the small plantar and palmar muscles.