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The following may be taken as a fair summary of what was known with reference to the circulation of the blood before Harvey’s great discovery. The blood was known to pass from the right ventricle and circulate through the lungs, returning in part at least to the left side of the heart. But the current opinion was, that not all the blood, but only the thicker and impurer part followed this course, while some of the purer still remained in the right side for further use, and a portion of it transuded through the minute apertures which imagination still feigned, though sense could not discern them, to the left side of the heart. Here as in an alembic the purer blood mixed, as was supposed, with a certain vital spirit with which the lungs had impregnated it, was transformed or distilled into that æther of twofold composition, one part sanguineous, one äereal, which it was the special office of the arteries to convey, informing the whole body with life, while the veins supplied the blood which subserved the humbler purposes of nutrition. Between these two sets of vessels there was supposed to be no direct communication—no circulation in the proper sense—but in each there went on a
