32. The arterial system springs from the heart by a single trunk, like a minute and hollow tree, with numberless branches. As these branches leave the heart they divide and subdivide, continually growing smaller and smaller, until they can no longer be traced with the naked eye. If, then, we continue the examination by the aid of a microscope, we see these small branches sending off still smaller ones, until all the organs of the body are penetrated by arteries. [Sidenote: 33. Successive undulations from the heart? Course of the arteries? Protection of the arteries? General location of the arteries?] 33. THE PULSE.--With each contraction of the left heart, the impulse causes a wave-like motion to traverse the entire arterial system. If the arteries were exposed to view, we might see successive undulations speeding from the heart to the smallest of the branches, in about one-sixth part of a second. The general course of the arteries is as far as possible from the surface. This arrangement is certainly wise, as it renders them less liable to injury, the wounding of an artery being especially dangerous. It also protects the arteries from external and unequal pressure, by which the force of the heart would be {116} counteracted and wasted. Accordingly, we generally find these vessels hugging close to the bones, or hiding behind the muscles and within the cavities of the body. [Sidenote: 34. Where do the arteries lie? If we apply the finger? Pulse? Where felt?] 34. In a few situations, however, the arteries lie near the surface; and if we apply the finger to any of these parts, we will distinctly feel the movement described, taking place in harmony with the heart-beat. This is part of the wave-motion just mentioned, and is known as the pulse. All are more familiar with the pulse at the wrist, in the _radial_ artery; but the pulse is not peculiar to that position, for it may be felt in the _carotid_ of the neck, in the _temporal_ at the temple, and elsewhere, especially near the joints. [Illustration: FIG. 30.--THE FORM OF THE PULSE.] [Sidenote: 35. The pulse as an index? Of what does it inform the physician? Instrument for recording pulsation?]