We measure the work of the heart by what we call blood pressure, about which we are hearing so much these days, so that it will be well to explain as clearly as possible just what is meant by it. The blood pressure really means the pressure of the blood within the large arteries. It could be measured with any ordinary pressure gauge, if it were not for the fact that we cannot very well cut into our bodies to apply gauges to the arteries. For this reason it has been necessary to invent means of finding out what the blood pressure is from the outside. The way it is done is to put a band around the arm, press this band down upon the arm until it squeezes the large arm artery shut, and then, by means of a suitable gauge, find out how much pressure was required. Of course, it is necessary to be able to tell when the artery has actually been squeezed shut, so that the determination of blood pressure in human beings is the work of an expert. Furthermore, blood pressure, as should be clear from what has already been said, varies with every heartbeat. It is at its maximum the instant the heart finishes emptying itself into the artery, and falls off steadily, reaching a minimum just before the next beat comes. The more elastic the arteries, the less difference there will be between the maximum and the minimum blood pressure. The reason for this will be clear when we think that if the arteries were entirely rigid there would have to be a very high pressure during the time the heart was actually beating to force the blood out through the capillaries, but that between beats the pressure would fall off to zero. The more elastic the arteries are, the more nearly do they exert a steady pressure on the blood within them, and so the less will be the difference between the maximum and the minimum pressure. When the physician determines blood pressure, he really determines both the maximum and the minimum pressure, in order that he may be able to judge whether or not the arteries are as elastic as they should be. High blood pressure just by itself might not mean much more than that the heart was beating more rapidly than it should, but a high maximum pressure and a low minimum pressure means nonelastic arteries. This in turn means that the blood is forced through the capillaries in jets rather than in a steady stream, and we may judge of the importance of having a steady flow through the capillaries when we recall the well-known medical proverb that “a man is as old as his arteries.” It is an actual fact that the chronological age of an individual need not have much to do with his physical age. If his arteries continue elastic over a long period of years, he will be physiologically youthful, while if his arteries become rigid he will be physiologically aged, no matter how few his actual years upon earth may have been. Unfortunately we do not know as much as we would like to about the causes of loss of elasticity in the arteries.