The New Student's Reference Work/Circulation of Blood
Circulation of Blood, the course of the blood in its round from the heart back again. A simple case of circulation is illustrated in the crayfish, where the heart consists of a single chamber with muscular walls. When it contracts, the blood is sent, in arteries, forward, backward and downward; on its return path it passes through the gills. In the clam there is a two-chambered heart. In fishes there are two chambers; in frogs and toads three; in the highest reptiles and all birds and mammals four chambers in the heart. The ancients believed that the arteries contained air, and only the veins blood. Galen (131-200), in the 2d century, demonstrated that both arteries and veins contain blood, but a direct connection between the two was not thought of. In the 16th century such a connection was believed in by Vesalius and others. In 1628 William Harvey published a book in which he maintained that the quantity of blood leaving the heart and the rate at which it leaves made a return to the heart necessary. This marks an epoch in physiology. He did not, however, see the minute vessels connecting arteries and veins. It remained for Malpighi, in 1661, and Leeuwenhoek, in 1669, to demonstrate, with the microscope, the existence of minute tubes connecting arteries and veins, and thus to show that the circulation takes place in a series of closed tubes. For further facts regarding circulation see Heart.