Finally the third opinion, that which now prevails most generally in France, consists in a union of the two preceding ones; the trunks and principal arterial branches are considered as incapable of acting upon the blood; but this property is attributed to the small arteries, and it is thought to be very great in the last divisions of these vessels. Thus, in this mixed opinion, the blood is carried by the sole influence of the heart in all the arteries of a considerable size; it is moved in part by the influence of the heart and in part by that of the parietes in the smaller arteries, and finally it is moved by the sole action of the parietes in the last arterial divisions. This action of the small vessels is also described as the principal cause of the course of the blood in the veins. In a question of this nature our opinion should be determined by experiments alone. This presents many points for elucidation. The first and the easiest to be decided is to ascertain if the arteries are or are not irritable. The problem was in some measure resolved in relation to the great arteries by the experiments of Haller and his disciples, by Bichat himself, and by those which M. Nysten has made upon man. For the purpose of being more perfectly convinced, I have sought, by all the known means, to develop the irritability of the arterial parietes; I have successively subjected them to the action of pricking instruments, of caustics and of galvanism, and I have never perceived any thing which resembled a phenomenon of irritability; and as those who maintain the irritability of the arteries pretend that if we do not perceive the contractions, it is because the experiments are made on too small animals, in whom the effects are but slightly apparent in consequence of the small diameter of these canals, I have repeated the experiment on large animals, on horses and asses, and I have never observed any other motions than the communicated motions. As the great arteries show no contraction, we ought to believe that the small ones would not; but as among the physiologists who reject the irritability of the arterial trunks, some like Haller, do not speak of the branches, others accord to them contractility, it becomes necessary to test this question by experiment; now these small vessels, like the larger ones, remain perfectly immoveable under the action of the scalpel, caustics and a stream of galvanic fluid.