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fels, the blood would be brought back to the heart without bene- 

 fiting the bodj ; whereas we may be allured that the blood is cir- 

 culated through the vellels for no other purpofe than to dillribute 

 nourilliment to every part. 



If we cut open an artery or vein of any animal, out of which 

 the blood is emptied, we fliall find the infide of it as clean and 

 \\hite as if no blood had ever been in it ; which I think is fufficient 

 proof, that thefe veflels are formed in fuch a manner as to retain 

 the thinner juices of the blood in every part, except where they 

 are divided into the moft minute ramifications. Nor will this feem 

 ftrange to us, if we confider thole fowls which fwim on the water, 

 and there leek their food ; for we know, that how long foever they 

 remain in the water, or how often foever the water covers their 

 feathers, yet thofe feathers do not at all imbibe the water. 



I have llripped off the very thin membrane, or internal coat of an 

 artery, and placed it before the micro fcope; when I faw, with ad- 

 miration, that it was compofed of an inconceivable number of ex- 

 ceffively thin parts, interwoven one among another, and linked to- 

 gether in the manner of a net : and when I examined the other 

 coat of the artery, which covered the laft mentioned, I faw that the 

 parts of which it was compofed, were carried round about the ar- 

 tery ; fo that the ftrength of fuch coat confifted in its being capa- 

 ble of great extenfion and contrattion in breadth, more than in 

 length. 



Seeing this, I concluded that every time the blood is driven from 

 the heart, all the arteries expand themfelves, which mull occafion 

 a pulfation throughout all thofe arteries ; and as by this means they 

 are every time ftretched beyond their ufual width, they are fo 

 formed and conllituted as immediately afterwards to contrail 

 themfelves, which contraction promotes the propullion of the 

 blood in thofe arteries, until it i-eaches thofe vellels we call veins. 



But the contrary, in my opinion, takes place in the blood-vellH 

 which we call a vein ; for a vein, when emptying a part of its con- 



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