Irritability does not exist then in the large or the small arteries. Respecting the last arterial divisions, as the vessels which form them are so small that they cannot come under the cognizance of the senses, at least in a state of health, no one can affirm or deny that they are irritable. Yet from analogy we ought to conclude, that they have no sensible motion. In cold-blooded animals, in fact, it is easy to see the blood circulating in these vessels, and even passing into the veins; now the vessels themselves appear to be completely immoveable. As the arteries cannot act upon the blood by contracting in the manner of muscles, must we conclude that they have no action upon this fluid, and that they are in relation to it nearly like inflexible canals? I am very far from thinking so. If in fact the arteries had no influence upon the blood, this fluid, moved by the sole impulse of the heart, would, from its incompressibility, be alternately in motion and at rest. This is indeed what Bichat thought, and what he has advanced in his other works; it is what has been since maintained in a more formal manner by Dr. Johnson of London. It is however very easy to prove that it is not in this way that the blood is moved in these vessels. Open a large artery in a living animal, and the blood will escape in a continuous jet, but by jerks; open a small artery, and the blood will flow out in a continuous and uniform jet. The same phenomena take place in man if the arteries are opened, either by accident or in surgical operations. The heart being unable to produce a continuous flow, since its action is intermittent, it must be then that the arteries act upon the blood; this action can only be the disposition which they have to contract, and even to obliterate their cavity entirely. Bichat thought that his tendency to narrowing was not sufficient in the arteries to expel the blood contained in their cavity. He maintains that the vessel does not contract upon itself only when the blood has ceased to distend it. If it were so, the arteries would be equivalent to inflexible canals, and the course of the arterial blood would not be continuous; but we can easily demonstrate that the force with which the arteries contract is more than sufficient to drive out the blood that they contain.