74 CIRCULATION OF THE BLOOD 



The last writer whose work we have to consider is 

 Anthony van Leeuwenhoek, another Dutchman, whose life 

 covered nearly a century, for he lived from 1632 to 1723. 

 Leeuwenhoek was a man in a humble walk of life who 

 never rose above the station of usher to a court in a small 

 Dutch town. He owed nothing to any teacher or any 

 university, and spoke and wrote no language but his own. 

 In his youth there had been in Holland a widespread 

 interest in Optics, and he developed remarkable skill 

 as an amateur lens maker. His lenses got better and 

 better, until finally he was making the best high power 

 lenses in Europe. 



At the same time Leeuwenhoek developed great talent 

 as an observer and as a shrewd interpreter of what he 

 observed. He did not publish anything until he was over 

 forty. In 1674, however, a specimen of his work was 

 sent to the Royal Society. The importance of it was at 

 once perceived, and it was translated and appeared in the 

 transactions of that learned body. From then on he 

 corresponded regularly with the Society, who printed all 

 or nearly all his work. 



At first Leeuwenhoek did not believe in the circula- 

 tion of the blood. He remained sceptical long after most 

 better informed and better educated men had become 

 convinced. He retained this attitude as late as 1686, 

 when, however, he began to waver. By 1688 he had 

 completely changed his opinions, for, in that year, owing 

 to the perfection that his microscopes had then reached, 

 he clearly saw not only the systemic capillary circulation, 

 but also the actual passage of the blood corpuscles 

 through the minute capillaries. He described the 

 capillary circulation in the web of the frog's foot, in 

 the mesentery of the same animal, and in the tails 

 of young fish (Plate VIII. Fig. 2). He described and 



