From all that has been said, it follows, I think, very evidently, that in the pulsation of the arteries, the heart is almost the only power that puts the fluid in motion; that the vessels are then passive, that they obey the motion that is communicated to them, but that they have none of themselves dependant at least on their vitality. Thus nature has chosen for the arterial texture one of those of the economy, in which life is the least evident; as the heart is remarkable for its vital properties, the arteries are remarkable for the absence of them. They must be ranked with the cartilaginous, fibrous, fibro-cartilaginous textures, &c. It is that they may not disturb the unity of impulse by their motions, that nature has thus formed the arteries. Suppose that they had the same vital forces as the intestines; what would become of life? Any convulsive contraction a little too strong in the aorta or in the great trunks, by contracting their caliber too much, would arrest the circulation, and produce the most serious effects by agitating it in an opposite direction to the heart. In the intestinal canal, this phenomenon only produces vomiting. It would produce death suddenly in the arterial system. The more attentively we examine, the more we shall be convinced of the necessity of having but one agent of impulse for the arterial system, and of having this system inert, so that it cannot be able to arrest the course of the blood. I do not say that the arteries can never contract from the vital influence; the skin which is not irritable, wrinkles by cold. But the cases are very rare, in which the arteries contract. When they exist they cause an inequality of the pulse on the two sides; an inequality rarely noticed in diseases. _Of the limits of the action of the Heart._ The heart is then the essential cause of the pulse; it is this which puts every thing in action in the arterial motion. Many authors have overrated its influence; they have thought that its impulse was sufficient to produce, not only the arterial motion, but also that of the general capillary system, and even that of the veins; so that the contraction of the left ventricle alone is the cause, according to them, of the long course the blood runs from it to the right ventricle. But it is incontestably proved, as we shall see, that when this fluid has arrived in the general capillary system, it is absolutely beyond the influence of the heart, and that it moves only by that of the tonic forces of the small vessels, and therefore for a stronger reason, the left ventricle has no influence in the venous system. It is in this respect that the authors, of whom I have spoken, have erred, and not under that of the impulse that they have admitted in the arterial system on the part of the heart.