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a vefTel, as will admit a grain of fand to pafs through it, is twenty- 

 live times thinner than fine poll paper, how much thinner mull be 

 the coat of a vein which will only admit the ten thoufandth part of 

 a grain of fand to pafs through it ? Therefore, if thefe very fmallert 

 blood-vclfels are provided with coats, fuch coats mull be fo thin, 

 open, or fpongy, that the very thinnell watery parts of the blood 

 can as eafily pafs through them as water through a fine fieve. 



After this, I took one of thofe fifli called a Jack, which I placed 

 fo that about the breadth of a finger of its tail was above the water, 

 and, upon examining the extremities of this tail fin b}' the micro- 

 fco])e, I could not perceive the leall motion in the blood in that 

 part ; \\ hence I judged that thofe parts of the tail were mortified, 

 for the filh, from its appearance, feemed to me to have been caught 

 fome da}'s. But when I examined the tail fin, nearer to the body, 

 I fiiw the blood llowly moving in an artery, and, a little nearer to 

 the body, there ilfued from this artery a fmall branch, through vvhich 

 the blood was carried with a very fwift current, and this branch, 

 taking a fmall bending, carried the blood into a vein which brought it 

 back towards the heart. Let G H I, in Jig. 15, rep relent this artery, 

 in vA'hich the blood from G to H was driven forward with its ufual 

 fwiftnefs, and from H to I its motion was very flow ; H K M, is a 

 fmaller vellel, through which the blood was carried from this artery 

 into the vein L M N, at the point N, and in that vein conveyed 

 back to the heart. 



After this, 1 bought a parcel of Trout, which had been caught in 

 the river Maes two or three days before, and among the reft was 

 one, not quite ieyen inches long, which, upon view of its fcales, I 

 judged to be be about five years old. This Trout, whofe tail fin was 

 a little injured at the extremity, I placed with its tail about a finger's 

 breadth: above llie water; and, when it was become quiet, I viewed 

 the tail by the microfcope, and I could not difcover any circulation 

 at the extremity, but, examining it a little nearer to the body, I faw 

 the blood in the arteries and veins ftagnating and coagulated ; and I 



