Of the Circulation of the Blood. 19 
able to go through the operation, but even make it almoft 
impoffible for you to fix him to your mind. Then fix 
his hind legs as before directed by the pegs p, q. If now 
you find him not quite faft, you may make him fo, by 
pulling out one of the pegs at a time, and {training the 
firing a little tighter, after which replace the peg. The 
ends of the firings which confine his arms and legs, are 
reprefented in the figure by the letters r, f, t, v, and the 
frog lying upon his back. 
The objedl being thus extended, and fafiened on the 
frame, as above directed, open the fkin of the belly, 
from near the anus to the throat, in the direction of the 
dotted line I K, by firft juft entering the point of a very 
fharp penknife at I, through the firft fkin only, taking 
care not to touch the fecond fkin, and let the incifion 
be no longer than the little ftreight ftroke I w, in which 
thruft the probe or diredtor, %• 47. almoft up to his 
throat, with the curf x y upwards, in which curf you 
may run the point of a pair of fciffars, without being in 
any danger of cutting any other part of the fubject, and 
thereby open the upper fkin from I to K, then turn the 
diredtor Tideways from K to L, and from I to M, and 
give it a little lhip in that direction, both at the top and 
bottom. Stick a fifh-hook, with the barbs filed off, into 
each corner of the fkin, firft having put the firings N O, 
to which the hooks are tied, through any two of the 
holes in the frame, as at P and Q. Then by gently 
pulling the firings N O,. the fkin will readily ftretch out 
into the direction of a fquare flap, as reprefented in the 
figure by the letters L M R S, and the three dotted lines' 
which furround them. 
If now you fix the frame to the microfcope, you may 
place any part of this flap before the magnifier, either 
by flipping the fquare fteel bar G H backwards and 
C 2 forwards 
