If, by means of a microscope, we examine the transparent web of a frog's foot, we can trace the route of the blood. [Footnote: With small splints and twine, a frog's foot can be easily stretched and tied so that the transparent web can be placed on the table of the microscope.] It is an experiment of wonderful interest. The crimson stream, propelled by the heart, rushes through the arteries, until it reaches the intricate meshes of the capillaries. Here it breaks into a thousand tiny rills. We can see the disks winding in single file through the devious passages, darting hither and thither, now pausing, swaying to and fro with an uncertain motion, and anon dashing ahead, until, at last, gathered in the veins, the blood sets steadily back on its return to the heart. THE CIRCULATION [Footnote: The circulation of the blood was discovered by Harvey in 1619. For several years, he did not dare to publish his belief. When it became known, he was bitterly persecuted, and his practice as a physician greatly decreased in consequence. He lived, however, to see his theory universally adopted, and his name honored. Harvey is said to have declared that no man over forty years of age accepted his views.] consists of two parts--the _lesser_, and the _greater_. FIG. 42. [Illustration: _Diagram illustrating the Circulation of the Blood._-- MARSHALL. A, _vena cava descending (superior);_ Z, _vena cava ascending (inferior);_ C, _right auricle;_ D, _right ventricle;_ E, _pulmonary artery;_ F P, _lungs and pulmonary veins;_ G, _left auricle;_ H, _left ventricle;_ I, K, _aorta._] 1. _The Lesser Circulation_.--The dark blood from the veins collects in the right auricle, and, going through the tricuspid valve, empties into the right ventricle. Thence it is driven past the semilunar valves, through the pulmonary artery, to the lungs. After circulating through the fine capillaries of the air cells contained in the lungs, it is returned, bright and red, through the four pulmonary veins, [Footnote: It is noticeable that the pulmonary set of veins circulates red blood, and the pulmonary set of arteries circulates dark blood. Both are connected with the lungs.] to the left auricle. 2. _The Greater Circulation_.--From the left auricle, the blood is forced past the bicuspid valve to the left ventricle; thence it is driven through the semilunar valves into the great aorta, the main trunk of the arterial system. Passing through the arteries, capillaries, and veins, it returns through the venæ cavæ, ascending and descending, gathers again in the right auricle, and so completes the "grand round" of the body. Both these circulations are going on constantly, as the two auricles contract, and the two ventricles expand simultaneously, and _vice versa_.