ceived 375 letters and papers." "The works themselves, except that they lie in the domain jf natural history, are disconnected and appear in no order of systematized study. The philosopher was led by what transpired at any moment to lead him."
In 1686 he observed the minute circulation and demonstrated the capillary connection between arteries and veins. This was perhaps his most important observation for its bearing on physiology. It must be remembered that Harvey had not actually seen the circulation of the blood, which he announced in 1628. He assumed on entirely sufficient grounds the existence of a complete circulation, but there was wanting in his scheme the direct ocular proof of the passage of blood from arteries to veins. This was supplied by Leeuwenhoek. Fig. 8 shows one of his sketches of the capillary circulation. In his efforts to see the circulation he tried various animals; the comb of the young cock, the ears of white rabbits, the membraneous wing of the hat were progressively examined. The next advance came when he directed his microscope to the tail of the tadpole. Upon examining this he exclaims:
This description shows that he fully appreciated the course of the minute vascular circulation and the nature of the communication between arteries and veins. He afterwards extended his observations to the web of the frog's foot, the tail of young fishes and eels.
In this connection it should be remembered that Malpighi, in 1661, observed the flow of blood in the lungs and mesentery of the frog, but he made little of it. Leeuwenhoek did much more with his discovery, and gave the first clear idea of the capillary circulation. Leeuwenhoek was also anticipated by Malpighi in reference to the microscopic structure of the blood. (See also under Swammerdam.) To Malpighi the corpuscles appeared to be globules of fat, while Leeuwenhoek noted that the blood discs of birds, frogs and fishes were oval in outline and
