INTRODUCTION OF THE MICROSCOPE 85 



directed his microscope to the tail of the tadpole. Upon 

 examininiij 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 turninsr, 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 savv^ 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 Vvhen 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 vounsr 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 



