94 THE FROG 



For injecting, the writer prefers to use a Luer syringe 

 of about 10 c.c. capacity; it should be provided with a me- 

 tallic cannula or " needle " beveled at the tip so as to present 

 a tapering point, not too sharp. It is important that the 

 cannula should be of proper caliber to fit accurately into the 

 truncus arteriosus of an average-sized frog; it is well to pro- 

 vide several different sizes, so that a selection may be made. 

 With forceps lift up the tip of the ventricle, and with fine 

 scissors make an oblique incision 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 in- 

 stead. While making the injection, with forceps hold the 

 parts firmly in place about the cannula; a ligature is un- 

 necessary, since the valves of the truncus arteriosus prevent 

 the escape of the injection mass after the cannula is removed. 

 Inject with slow and steady pressure as much of the injec- 

 tion mass as the arteries will stand without bursting a 

 point that must be learned by experience. Some operators 

 prefer to use a sharp cannula which is thrust directly into 

 the ventricle without making a preliminary incision; this 

 method requires care in order to avoid perforating the wall 

 of the truncus. 



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 2 per cent, for- 

 malin, 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 particularly the cceliaco-mesenteric artery and its 



