In 1690 Leuwenhoek (1632-1723), who had been making observations on the larvæ of frogs and other small animals, was able to see with his improved microscope the movements of the blood in the small vessels, and gave the important testimony of his observations. In 1687 Cowper saw the passage of the arterial into the venous current in the mesentery of a cat. The capillary connection between the two vascular systems was first demonstrated by Marchetti, but was best shown by Ruysch (1638-1731), professor at Amsterdam, the famous inventor of minute injections, who greatly advanced anatomy by the formation of collections, one of which was brought into Russia by Peter{ 159} the Great at an expense of about seventy-five thousand dollars. The Russian transporters of the collection, however, drank the alcohol in which many of the preparations were preserved, and a portion of the specimens was thus ruined.
Further illustration and amplification of Harvey's views came from various sources; the last, perhaps, from Nich-olaus Steno (1638-1686), who was first a professor in Copenhagen, then a bishop and peripatetic converter of heretics. Steno first proved the heart to be a muscle that contracts actively and expels the blood. The duct that bears his name was discovered during his residence in Leyden or at Amsterdam. His name is written also "Stenson."
While ancient anatomists were able to describe in a general way the form of the lungs, their location, consistency, the ring-like structure of the trachea, and the first division of the bronchi, they did not go farther, but blindly accepted the prevalent theory that the bronchial tubes anastomosed with the terminal pulmonary veins, and that in this way atmospheric fluid was conveyed from the respiratory organs into the heart. On such vague and erroneous data was constructed the theory that the air was drawn into the lungs by the heat of the heart, which was the reservoir of the vital spirits; that in penetrating through the smaller tubes it was rarefied, its thinnest part passing into the heart, where it served as material for the formation of the vital spirit, its grosser part being exhaled. In other words, respiration was supposed to have two purposes one to refresh the lungs, which, being porous and inflammable, would otherwise take fire from the heart, or focus of animal heat; the other to furnish the pneuma, or ether, which was employed by the heart in the formation of animal spirits. Harvey's discovery upset all this, in great measure.