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it was above a thoufand times larger than thofe velTels in which he 

 had feen the circulation of the blood at my houfe. 



Hereupon I told him, that in this cafe a perfon might very eafily 

 be deceived ; and that the veiTel or veflels to which the names of ar- 

 teries or veins were given, might, in my opinion, be either one or 

 the other; for that I had myfelf, at times, feen various veflels in 

 which I concluded the blood had performed its circulation, and 

 which were fo large that they might be difcerned by the naked 

 eye, but that, in thofe inftances, I had always been miftaken ; for, 

 upon a more clofe examination, I had found that thofe feveral vef- 

 fels had arifen out of a branch of one and the fame blood-veflel. 

 To illuftrate this by an example : 



Let A B C D E F G, in fig. 7, reprefent a vein, Vv'hofe two 

 branches are both cut off at B and E, fo that we do not know that 

 each of thefe is a branch ifluing from the vein A. Thefe two vef- 

 fels, BCD, and E F G, we find again united by the veffel E F, as 

 the blood-velTels are often joined together ; and out of thefe vef- 

 fels, by fqueezing, pinching, or rubbing, we may extrad any blood 

 that might be left in them. Now it may very eafily happen, that 

 in extracting the blood, or by the ftrong injedlion of quickfilver or 

 wax, the valves (if any fuch there are in the veflels) may be broken, 

 and then the quickfilver or wax, fo inje6led, may be forced through 

 that part of the veflel marked B C F, and return from F to E ; 

 whereupon BCD will feem to be an artery, and E F G a vein, and 

 it will be thought that the circulation is performed through C F. 

 When velTels of this kind have formerly been viewed by me, I 

 have found myfelf to be miftaken in fuch concluflon ; for, upon 

 farther examination, I have found (as before mentioned) that both 

 the branches BCD, and E F G, arofe out of the fame vein, A. 



Again, fhould we meet with an artery, as reprefented in fig. 8, 

 I KL, into which we injedl quickfilver until the whole veflel I K L 

 is filled, lb that we may be certain that in this veflel the blood is 

 brought back from K to L towards the heart; neverthelefs, we 



