Royal Society. 451 
the vein at V had not been united with T, but had only lain with 
its aperture at V in the extravaiated blood, fo that the extrava- 
lated blood was only for a little fuckt up and conveyed thro' it ; 
Then M. Leeix)enboek faw a vein, wherein the motion of the 
blood feemed very uncommon, as for example, let Fig. i/^.abht ^n 
artery, wherein the blood is impelled with great velocity from a 
to by then b Cy whereby the blood is conveyed towards the heart, 
muft be called a vein ; and clofe by it lies another artery dccy 
thro* which the blood was conveyed from the heart from d to c^ 
now if the vein bCy be united with the artery de^ as isleen at c, 
and thus the blood be conveyed from c to Cy be Ihould be called a 
vein, and the blood coming to f, and being there transfufed into 
cey is the arterial blood, becauie it is conveyed thither from the 
heart, it being certain that ^ ^ ^ is an artery. 
Amongft the reft M. Lee^jcerihoek had a tadpole before him, 
wherein he could perceive no motion at all of the blood, how 
attentively foe ver he viewed it, whereof at firft there appeared 
no realbn, 'till upon contemplating this animal with the naked 
eye, he obferved that the fore-part of its body was contra6ted, by 
which he imagined that the heart was opprefled, fothat it could not 
force out the blood, and receive it back again ; whilfl he was con- 
fidering this, the little animal made a very ftrong niotion, beat- 
ing its tail about, and bending its body, by which it got clear of 
the oppreffion it was under 5 and upon viewing it again, he im- 
mediately perceived that the blood began to have a flow motion 
and impulfe in feveral veflels, which increafed fo, as at length to 
come to its proper motion, yet not with fuch velocity, as it would 
have had, if either the heart or body had not been opprefTed : 
The motion of the blood in thefe tadpoles exceeds what M. 
Leeiioenhoek ever faw in any fmall animal.^ Fig. 1 5. reprefents a 
tadpole arrived to fuch a bignefs, as that it could ufe its hinder 
legs 5 and the fore-legs were alfo difcernible, but yet covered 
with the fkin; at laft M. Lee'iJOenhoek efpied an artery, thattho* 
it was fo fmall, as that he judged that one little globule of blood 
could pafs through it, yet out of fuch a branch there ftill pro- 
ceeded two other branches, in each of which the blood flowed, 
yet farther afunder and flower, than it had done before it came 
into the feparated veffels 5 after this he fixt his eye upon the great 
artery and vein, which were fo clofe to each other, that there 
was not above the diftance of the fourth part of the breadth of a 
hair between them j and it happening that the animal, whilft he 
viewed it, moved its head upwards, and tail downwards, the 
blood run upwards in the artery and downwards in the vein, and 
L I 1 2 that 
