68 THE FROG 
proper caliber to fit accurately into the truncus arteriosus of an 
average-sized frog; it is well to provide several different sizes, 
so that a selection may be made. With fine forceps lift up the 
tip of the ventricle, and with fine scissors make an oblique in- 
cision through the wall of the ventricle about midway between 
its base and apex; through this opening insert the tip of the 
cannula into the cavity of the ventricle, thence into the truncus 
arteriosus ; some care and skill is required to avoid forcing it into 
the auricles instead. While making the injection, with forceps 
hold the parts firmly in place about the cannula; a ligature is 
unnecessary, since the valves of the truncus arteriosus prevent the 
escape of the injection mass after the cannula is removed. In- 
ject with slow and steady pressure as much of the injection mass 
as the arteries will stand without bursting—a point that must be 
learned by experience. 
In case a metallic cannula of the proper size cannot be secured, 
a cannula may be made by drawing out a piece of glass tubing 
after heating it in a flame. This may be fastened to the syringe 
with a piece of rubber tubing, but on account of the flexibility of 
the connection such a cannula is not so easy to use. 
It is advisable to keep the frogs in moderately warm water 
for a short time immediately before and after the operation. 
They should be preserved for a few days in weak formalin 
(2 per cent.), and afterwards changed to 70 per cent. alcohol. 
Permanent preparations of some of the best injections may be 
made, with the principal arteries dissected out, and showing par- 
ticularly the cceliaco-mesenteric artery and its branches since these 
often fail of satisfactory injection. Such a preparation may be 
pinned out on a plate of wax and exhibited in a jar of alcohol. 
3. The veins. Frogs will be needed especially for the study 
of the venous system, but the veins need not be injected artificially 
since the blood affords a natural injection of the veins by collect- 
ing in them after death, leaving the arteries empty. The speci- 
mens should be killed with ether and preserved in weak formalin 
(2 per cent. or 3 per cent.) ; an incision should be made through 
the ventral wall of the abdomen to allow the formalin to enter the 
body cavity. They should be rinsed for a few hours in water 
before being used for dissection. 
