About the beginning of the seventeenth century William Harvey became convinced that there must be fine vessels leading across from the smallest arteries to the smallest veins and that the blood must pass through these. He came to this conclusion without ever having seen these small vessels, since at that time there was no microscope by which such tiny structures could be seen. Before Harvey’s time it was supposed that the blood ebbed and flowed in the arteries and in the veins. He showed that the blood flows in one direction constantly, leaving the heart by way of the arteries and coming back into it by the veins. Harvey was, therefore, the discoverer of the circulation, one of the most important discoveries in physiology. After the microscope was perfected the tiny tubes connecting the finest arteries with the finest veins were made out. They were found to be very small in diameter and to have very thin and delicate walls. They were also found to be extremely numerous. The finest subdivisions of the arteries that can be seen with the naked eye are scattered very thickly through all the tissues of the body which have a blood supply, and they go on subdividing microscopically, so that the finest vessels are scattered more and more thickly through the mass of living substance. The very finest of all, which are the tubes connecting the smallest arteries with the smallest veins, are called _capillaries_, from a Latin word meaning a hair, to indicate their very small size. They are so close together in most parts of the body that it would be difficult to thrust a [Illustration: Photo, Paul Thompson THE TEST FOR BLOOD PRESSURE] [Illustration: Photo, Paul Thompson PART OF PROCESS OF URINALYSIS INDICAN TEST] [Illustration: A NETWORK OF CAPILLARIES The artery _a_ and vein _v_ (highly magnified). (From “The Human Mechanism,” Hough and Sedgwick)]