THE subject of blood-pressure is one of great interest both to the physiologist and the clinical physician. By blood-pressure is meant the pressure which the blood exerts on the interior of the heart and blood-vessels, but it is chiefly with the vascular blood-pressure—arterial, capillary, and venous—that the physician deals. Our conception of intravascular pressure is facilitated by considering what happens when an aperture is made in an artery, capillary, or vein of a living animal. In the case of the artery the blood squirts out with considerable force, the height of the jet measuring the pressure exerted on the interior of the vessel. Experiment shows that the pressure falls slowly from the heart to the region of the smallest arteries, or arterioles, where there is a considerable fall, the pressure in the capillaries and veins being comparatively low; in the large veins opening into the right heart it may, indeed, be minus, owing to the suction action of the thorax, and hence when these veins are cut air may actually be sucked into the blood-stream.