INTRODUCTION OF THE MICROSCOPE 85 
directed his microscope to the tail of the tadpole. Upon 
examining this he exclaims: 
‘““A sight presented itself more delightful than any mine 
eyes had ever beheld; for here I discovered more than fifty 
circulations of the blood in different places, while the animal 
lay quiet in the water, and I could bring it before my micro- 
scope to my wish. For I saw not only that in many places 
the blood was conveyed through exceedingly minute vessels, 
from the middle of the tail toward the edges, but that each 
of the vessels had a curve or turning, and carried the blood 
back toward the middle of the tail, in order to be again con- 
veyed to the heart. Hereby it plainly appeared to me that 
the blood-vessels which I now saw in the animal, and which 
bear the names of arteries and veins are, in fact, one and the 
same; that is to say, that they are properly termed arteries 
so long as they convey the blood to the furtherest extremities 
of its vessels, and veins when they bring it back to the heart. 
And thus it appears that an artery and a vein are one and 
the same vessel prolonged or extended.” 
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 afterward 
extended his observations to the web of the frog’s foot, the 
tail of voung fishes and eels. 
In connection with this it should be remembered that 
Malpighi, in 1661, observed the flow of blood in the lungs 
and in the mesentery of the frog, but he made little of the 
discovery. Leeuwenhoek did more with his, and gave the 
first clear idea of the capillary circulation. Leeuwenhoek 
was anticipated also by Malpighi in reference to the micro- 
scopic structure of the blood. (See also under Swammer- 
dam.) To Malpighi the corpuscles appeared to be globules 
of fat, while Leeuwenhoek noted that the blood disks of 
birds, frogs, and fishes were oval in outline, and those of 
