4 Of the Circulation of the Blood. 
and fubdivided into their fmalleft component parts, be¬ 
fore they can find a paffage. 
Providence has been furprifingly careful in the difpofi- 
tion of the veins and arteries, for thefe laft, which convey 
the blood to the extremities of the animal, continually 
lelTen their diameters in their progreflion, and divide into 
fmalier branches. At which divifion, the globules rufli 
againft an angle, which as it were caufes them to recoil 
upon thofe immediately behind, before they can readily 
feparate into the two fmalier branches C D, of the artery 
A B, fig. 31. in which the blood flows upwards from B 
to A, towards the extremity ; and on the contrary in its 
return back from the extremities to the heart, their dia¬ 
meters increafe, and thofe fmalier veffels are continually 
uniting into larger, as in the foregoing, fig. 31. the 
branches C and D join their currents in the vein E F, 
till at lafi: all their llreams fall into one, at every fuch 
conjunction of two branches, as at E, and their ftreams 
violently rufh againft each other, by which means unna¬ 
tural cohefions are prevented. 
The microfcope affords us an ample view of the 
yeins and arteries, the latter of which is very diftinguifh- 
able by a protrufion of the blood, at each contraction of 
the heart, then a ftop, and then a new protrufion, con¬ 
tinually fucceeding each other, whilft in the veins it rolls 
on with inexpreflibie rapidity. 
The ingenious Mr. Leeuwenhoek hath told us, that 
with great admiration he faw in the utmoft extremities 
of a very fmall fifti’s tail, how the larger arteries were 
divided into the fineft veffels c , and many of the fmall 
veins, which returned from the faid extremities, met to¬ 
gether in a larger vein; that there was fuch an agita¬ 
tion of that blood, which flowed from the larger arteries, 
towards 
Arc. Nat. tom. iv. p. 107. 
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