The Human Heart
If we regard the construction of the blood-vessels, and other parts of the circulating system, we find that they are constructed entirely on physical laws. The Heart is the mover which propels the blood, and, after having given the stroke, its fibres become relaxed, to receive a fresh supply. In this case it is important that the fluid should not again regurgitate into its cavities; and to prevent that, a system of valves, not thicker than paper, has been contrived. Here we see a design identical with that pursued by man in the construction of his pump, or even, in some cases, of his floodgates. The only difference between the work of man and the work of Nature is, that the latter is executed in a manner so superior, that man feels that he sinks into insignificance beside the Creator.
That wonderful machine, the Heart, goes night and day, for eighty years together, at the rate of 100,000 strokes for every twenty-four hours, having at every stroke a great resistance to overcome. Now, each ventricle will contain at least one ounce of blood; the heart contracts 4000 times in an hour, from which it follows that there pass through the heart every hour 4000 ounces, or 350 pounds of blood. The whole mass of blood is said to be about twenty-five pounds; so that a quantity equal to the whole mass of blood passes through the heart fourteen times in one hour, which is about once in every four minutes.