121 
and Circulation of the Blood. 
ficult to be allured of than the former; for 
when the VelTels before us are extremely 
fmall, it is not always eafy to diftinguilh 
which of them are Veins and which are 
Arteries. 
The larger Arteries are indeed dillinguifh- 
able by a Protrulion of the Blood at each 
Contradion of the Heart, then a Stop, and 
then a new Protrulion, which may plainly 
be feen continually fucceeding one another: 
whereas the Current paffes through the 
Veins with an equal and unintermitting 
Stream. But in the more fine and extreme 
Branches of the Arteries this Difference is 
not perceivable. 
The tranfparent Membrane between a 
Frog's hind-foot Toes is the Objed moll 
commonly employed for viewing the Current 
and Circulation of the Blood ; and in this, if 
well expanded, it may be feen fairly and dif- 
tindly, both in the Veins and Arteries, in 
the Manner reprefented Plate XI. Fig. I, 
A. A. two Toes of a Frog’s hinder Foot. 
B. the thin Membrane between the Toes 
extended. 
C. C. C. the Trunks of the Arteries. 
D. D. the Trunk of a Vein. 
E. E. E. Arteries and Veins in the fine 
Membrane, with the Blood-Globules circu¬ 
lating through them. 
The 
