35. Since the heart-beat makes the pulse, whatever affects the former affects the latter also. Accordingly, the pulse is a good index of the state of the health, so far as the health depends upon the action of the heart. It informs the physician of the condition of the circulation in four particulars: its rate, regularity, force, and fullness; and nearly every disease modifies in some respect the condition of the pulse. A very ingenious instrument, known as the sphygmograph, or pulse-writer, has recently been invented, by the aid of which the pulse is made to write upon paper its own signature, or rather to sketch its own profile. This instrument shows with great accuracy the difference between the pulses of health and those of disease. In Fig. 30 is traced the form of the pulse in health, which should be read from left to right. That part of the trace {117} which is nearly perpendicular coincides with the contraction of the ventricles; while the wavy portion marks their dilatation. [Sidenote: 36. What are the veins? How do they form? What do they resemble?] 36. THE VEINS.--The vessels which convey the blood on its return to the heart are the veins. They begin in the several organs of the body, and at first are extremely small; but uniting together as they advance, they constantly increase in size, reminding us of the way in which the fine rootlets of the plant join together to form the large roots, or of the rills and rivulets that flow together to form the large streams and rivers. In structure, the veins resemble the arteries, but their walls are comparatively inelastic. They are more numerous, and communicate with each other freely in their course, by means of interlacing branches. [Illustration: FIG. 31.--THE VALVES OF THE VEINS.] [Sidenote: 37. Valves in the veins? What are they? Their position? Experiment with the cord?] 37. But the chief point of distinction is in the presence of the valves in the veins. These are little folds of membrane, disposed in such a way, that they only open to receive blood flowing toward the heart, and close against a current in the opposite direction. Their position in the veins on the back of the hand may be readily observed, if we first obstruct the return of blood by a cord tied around the forearm or wrist. In a few minutes the veins will appear swollen, and upon them will be seen certain prominences, about an inch apart. These latter indicate the location of the valves, or, rather, they show that the vessels in front of the valves are distended by the blood, which cannot force a passage back through them. [Sidenote: 38. What will be proved by the experiment? What inference is drawn?]