This resistance of the arterial texture, so different from that of the venous, is a necessary consequence of the situation of the heart at the origin of the arteries. In fact, this organ driving the blood with force into their tubes, should find there a force capable of resisting the greatest efforts of which it is susceptible, when its sensible organic contractility is raised to a high point. This is the great advantage of the arterial texture. What would become of the circulation and all the functions that depended upon it, if the least cause which increased the force of the blood could dilate the parietes of the arteries beyond the ordinary degree? It was necessary that their texture should render these parietes independent of the different degrees of motion of the fluid that circulates in them; whence it follows that a fleshy heart and resisting arteries are two things inevitably connected. If nature had doubled the energy of the heart, she would have doubled also the arterial resistance. On the other hand, they would have had but little resistance, if there had not been an agent of impulse at their origin; this is precisely what happens in the hepatic portion of the vena porta, which by its distribution is analogous to the arteries. Why is the pulmonary artery thinner and less resisting than the aorta? Because the right ventricle being less fleshy, is not capable of so great efforts. From what has been said, it appears, that the external arterial membrane resembles the fibrous organs, which, as we shall see, are characterized by an extreme resistance. But if we observe on the other hand that this membrane can be broken, raised by layers and scales in dissection, that it is elastic and even dry, if I may so say, whilst the fibrous organs are compact, form a solid body, resisting, but softer and more elastic, we shall be convinced that this external membrane is exclusively peculiar to the arteries; that it has no relation with the other systems, but forms a distinct and separate texture in the economy. The structure with regular fibres, is the only circumstance that can, in my opinion, make us believe in the muscular nature of the arteries; but the ligaments and tendons are fibrous also; of what importance are the forms to the intimate nature? Now, can we say that this nature is the same, when the physical properties, when the extensibility and contractility of texture, when the vital sensibility and contractility are different? Besides, the action of different re-agents upon the arterial texture, proves clearly how much it differs from the muscular. There are then general phenomena common to all the solids; but different peculiar phenomena that are distinctive. We may satisfy ourselves of this, by comparing the following article with that which corresponds with it in the muscular system. _Action of different agents upon the arterial texture._