HARVEY. 33 as to force out of them any fluid they may happen to contain. By the relaxation of the same fibres, these cavities are in their turn dilated, and of course prepared to admit any fluid which may be poured into them. Into these cavities are inserted the great trunks, both of the arteries which carry out the blood, and of the veins which bring it back. The ar- teries arise from cavities called ventricles ; the veins pour their contents into cavities denominated auri- cles. By the successive contractions and dilata- tions of these several cavities of the heart, it has been calculated that all the blood in the body passes through the heart about once in four mi- nutes. Consider what an affair this is, when we come to very large animals ! The aorta (which is the name given to the chief artery) of a whale is larger in the bore than the main pipe of the water-works at London-bridge, and the water roaring in its passage through that pipe is inferior in impetus and velocity to the blood gushing from the whale's heart. To render this short account more precise, it must be observed, that with the apparatus men- tioned above, two distinct circulations are carried on. For besides circulating generally through the foody, the blood must come somewhere into conti- guity with the air, in order to purify it, and change its colour from dark to bright red. Hence the heart is, as it were, a double organ, having a double office to perform : of its four cavities, two are em- ployed to carry on the general circulation, while the remaining auricle and ventricle keep up the smaller circulation through the lungs, where the blood meets with the atmospheric air. D
