294. The heart projects the blood with a given force into the arterial tubes. The arteries in the living body are always filled to distension, and somewhat beyond it, by the quantity of blood that is in them. It has been shown that the elasticity of their coats is such as to give to them, even after death, the form of open hollow cylinders (274). During life they are kept in a state of distension by the quantity of blood they contain. By virtue of their elasticity they react upon their contents with a force exactly proportioned to the degree of their distension, that is, with a force at least adequate to keep them always open and rigid. 295. These open and rigid tubes, already filled to distension, and somewhat beyond it, receive at every contraction of the heart a forcible injection of a new wave of blood. The first effect of the injection of this new wave into a tube previously full to distension, is to cause the current to proceed by jerks or jets, each jerk or jet corresponding to the contraction of the heart. And, accordingly, by this jet-like motion, the flow of the blood in the artery is distinguished from that in the vein, in which latter vessel the current is an equal and tranquil stream. 296. The second effect of this new wave is to occasion some further distension of the already distended artery, and accordingly, when the vessel is exposed in a living animal, and its action carefully observed, a slight augmentation of its diameter is distinguishable at every contraction of the heart. This new wave while it distends must at the same time slightly elongate the vessel; cause its straight portions to bend a little, and its curved portions to bend still more; and, consequently, in some situations, to lift it a little from its place, giving it a slight degree of locomotion;—and these two causes combined produce the pulse. When the finger is pressed gently on an artery, at the instant of the contraction of the heart, the vessel is felt to bound against the finger with a certain degree of force: this, as just stated, is owing to a slight distension of the vessel by the new wave of blood, together with a slight elongation of it, and a gentle rising from its situation.