130 Transactions of the Society. 



the circulation of the blood, but it was left to Leeuwenhoek to- 

 complete the discovery of the circulation, and he was the first to- 

 demonstrate it ante ocidos. Malpighi had, as well as Harvey, 

 suspected the existence of capillaries and of the circulation through 

 them, and Leeuwenhoek saw those capillaries and saw the blood 

 circulating through them, and demonstrated them and it to others ;, 

 and this was quite independent of Malpighi's work. This watch- 

 ing the circulating blood was a thing he never tired of, and his 

 method of compassing it was very ingenious. What Harvey had 

 inferred, Leeuwenhoek, with his simple instrument, made visilale in 

 1686, and his paper on "The true circulation of the blood," pub- 

 lished in 1688, is still delightful and useful reading. He first 

 tried the comb of a cock, then the ear of a rabbit, then a bat's 

 wing ; but these were not satisfactory. Then he tried a tadpole's 

 tail, and he saw what he wanted. He says: "A sight presented 

 itself more delightful than any that my eyes had ever beheld ; for 

 here I discovered more than fifty circulations of the blood in differ- 

 ent places, while the animal lay quiet in the water, and I could 

 bring it before the Microscope to my wish. For I saw, not only 

 that the blood in many places was conveyed through exceedingly 

 minute vessels, from the middle of the tail towards the edges, but 

 that each of these vessels had a curve, a turning, and carried the 

 blood back towards the middle of the tail, in order to be again 

 conveyed to the heart. Thereby it plainly appeared to me that the . 

 blood-vessels I now saw in this animal, and which bear the name 

 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 farthest extremities of its vessels, and veins when they 

 bring it back towards the heart. . . . And thus it appears, that 

 an artery and a vein are one and the same vessel prolonged or 

 extended." He noticed that in the capillaries only one globule — 

 as he calls the corpuscles — could pass at a time, and he also 

 noticed the compression it undergoes in the journey. He then 

 saw it in the tail of a little fish he caught accidentally with Ijis 

 tadpoles, and eventually in the tails of small eels, which were his 

 animals of choice afterwards, as they appeared to show it better 

 than any. He recognized that the minuteness of the capillaries 

 was necessary to ensure the equal and universal nutrition of the 

 body, and he noticed the change of colour in their blood, and said 

 it was due to the less bulk ; and he said that the fluid parts of the 

 blood permeated the thin capillary walls in order to supply nutri- 

 ment to the tissues. He also elucidated the effect of arrest of cir- 

 culation in a vessel w^ith a perspicuity which was astounding, and 

 he saw the actual formation of a minute new vessel below an 

 obstructed part. He recognized the part* the heart played in the 

 circulation ; he says : *' I never looked upon the Heart as the maker 

 of the Blood, but only as an Engine that caused the Blood to cir- 



