THE FROG 69 
4. The capillaries, and their relation to arteries and veins, are 
best demonstrated in the web of the foot of a living frog. For 
this purpose a young frog is best, since the web is thinner and 
contains less pigment than in an older specimen. Fasten the frog 
to a small piece of thin board by means of a cloth jacket which 
will not interfere with the circulation; spread out the toes of a 
hind foot so that the web will be stretched across a hole near the 
corner of the board, and tie them in that position with thread. 
Keep both upper and under surfaces of the web well moistened 
with normal salt solution, and cover a portion of the web, be- 
tween two digits, with a triangular piece of cover glass in order 
that it may be examined with high as well as with low power. 
The entire frog, excepting the foot, should be covered with a 
wet cloth. Provide a box of equal height with the stage of the 
microscope, and place the frog, stretched out on the board, on the 
box and the stage. Adjust the board so that the web of the frog’s 
foot is in position to be viewed with the microscope; the hole in 
the board will admit light reflected by the mirror. It may be more 
convenient to kill the frog (by pithing) ; it may then be pinned to 
the board. In such a preparation the circulation will usually con- 
tinue for several hours. 
The capillary circulation may also be demonstrated to good 
advantage in the mesentery of a frog killed by pithing. Cut 
through the body wall of one side of the abdomen and pull out a 
loop of the small intestine. Pin this out on a board in such a 
manner as to spread out the mesentery and exhibit in the same 
way as the web of the frog’s foot. 
D. The Respiratory System.—An entire specimen should be 
prepared with the lungs exposed and inflated. If blown up with 
air they may be hardened in that condition in formalin. 
The vocal sacs of the male may be injected with gelatin and 
exposed by removing the overlying integument. 
Permanent preparations of the hyoid apparatus, larynx and 
lungs, dissected out as directed in the text, are required. 
Separate lungs slightly inflated, then hardened in formalin and 
cut open so as to expose the interior, should be exhibited in small 
glass vials. 
