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a vein, and, if a perfon did not clearly fee the different courfes of the 

 vefl'els, he might concUide that here the circulation was performed 

 or compleatcd, and this I faw not only in the fmalleft velTels, but in 

 thofe which were ten times larger. This tranfverfe courfe, or cur- 

 rent of the veni'ls, I had often before obferved in terreftrial animals, 

 Avhilft I was endeavouring to difcovcr the conjun6lion of their arte- 

 ries and veins. But, as I became then well afllired, that this con- 

 jun6tion, or circulation, did not take place in the larger veflels, but 

 in the very fmallefl: of all, (for otherwife I am pcrfuaded, that all the 

 parts of the body could not be fupplied with nourifhment) and that 

 therefore I Ihould in vain attempt to difcover fuch circulation, I, fome 

 years ago, abandoned the farther inveftigation of it. For, if we now 

 plainly perceive, that the paflage of the blood from the arteries into 

 the veins of the tadpole, is not performed in any other tlian thofe 

 veffels, which are fo minute as only to admit the pallage of a fmgle 

 globule at a time, we may conclude that the fame is performed in like 

 manner in our own bodies, and in thofe of other animals. And if 

 fo, it mull be impofhble for us ever to difcover the padage from the 

 arteries into the veins in the human bod}^ or of any terreftrial aiii- 

 animal ; firft, becaufe a fingle globule, when lying in one of thofe 

 minute arteries, has not any vifible colour, and fecondly, becaufe, in 

 thofe veffels whicli are fo large as to come within our fight, the 

 blood has not any perceptible motion. 



The particles in the blood of frogs are (as before obferved) of 

 a flat and oval Ihape, and thefe, when \iewed fingly, appear, as I 

 may fay, colourlefs ; but wlien two are laid on on.e another, tliey 

 appear a little of a reddifh cafl ; and v>here tlirce are together, tlie 

 rednefs becomes deeper. This may be explained ^y Jig- S- '^vlierein 

 A reprefents one of thefe oval particles, whicii, in part, is covered 

 by another particle B ; and C is a third particle, covering a fmall 

 part of both A and B ; by which means the part at D, where tlie 

 particles are three-fold, acquires a ffill darker colour. But when 



