Longitudinally, the arteries are more capable of stretching, than they are transversely. We may be convinced of this, by drawing out these vessels, to place a ligature upon them in an amputated stump. By cutting upon a dead body a portion of artery, and drawing it in a contrary direction, it is evidently elongated. It is necessary in these experiments, to pay attention to the development of the folds. In fact, I have said, that this development of the folds performs a principal part in the elongation of the arteries situated in the parts that are dilated. It is evident that in the extensibility in a transverse direction, it is the circular fibres of the peculiar membrane that especially resist; that on the contrary, in the extensibility in a longitudinal direction, it is the common membrane that opposes the resistance, since there are no longitudinal fibres. It is not astonishing then that the first kind of extensibility should be less evident than the second. _Contractility._ It is necessary to consider it in a transverse and in a longitudinal direction. Considered in the first point of view, contractility is much more evident than extensibility. When the artery is no longer distended with blood, it contracts in a sensible manner. It is to this contraction, that the following phenomena must be referred; 1st. the umbilical artery and the ductus arteriosus, become like ligaments after birth, by the adhesion of their parietes which are contracted. 2d. If we make a ligature upon an artery, the whole portion comprised between this ligature and the first collateral branch, soon exhibits the same phenomenon, as is proved by the operation for aneurism. 3d. If we include a portion of the carotid between two ligatures, and afterwards empty it by a puncture, it suddenly loses half its caliber. 4th. In dogs in whom I have transfused blood in order to produce artificial plethora, I have observed the arteries to be almost double in diameter, to what they are in those of the same size, who had suffered great hemorrhage. Two animals of the same size, one killed by hemorrhage the other by asphyxia, exhibit the same difference. 5th. These experiments shew me satisfactorily the cause of a large and small pulse, a cause admitted moreover by most physiologists. The artery is certainly more or less large, according to the quantity of blood that fills it. There is a point of extension that it cannot pass; but it contracts often for the want of blood, so as to be as it were, but a mere thread. 6th. Though you may have opened but few bodies, you have no doubt been astonished, that in those of the same size, the arteries have often very different diameters. This arises wholly from what takes place at the moment of death. If, from the want of blood, the arteries are for a long time contracted, they remain in this state, as happens to the heart in death by hemorrhage, &c. This is so true, that arteries of