THE FROG 



CHAP. 



. art 



abdomen of a freshly-killed frog in the usual way, taking great care not 

 to injure the blood-vessels. Remove the middle portion of the 

 shoulder-girdle so as to expose the heart, 

 lay open the pericardium, and with a 

 single snip of the scissors cut off the pos- 

 terior half of the ventricle, allowing the 

 blood to escape freely. Pass a piece of 

 thread (not cotton), about six inches long, 

 round the heart, at about the junction of 

 the auricles and ventricle, and give it a 

 single loose tie, as shown in Fig. 25. 

 When the bleeding has ceased, fill the 

 medicine dropper, or syringe, with injec- 

 tion-mass and pass the narrow end of the 

 former, or the nozzle of the latter, through 

 the cut end of the ventricle into the conus 

 — take care not to push it into one of the 

 auricles instead — and tighten the thread so 

 as to keep it in place. Then squeeze the 

 cap of the medicine dropper, or push in the 

 piston of the syringe, and if the operation is 

 successful, you will see the blue injection 

 pass from the conus into the arterial trunks, 

 and thence into the various arteries of the 

 body. The contrast between the arteries, 

 filled with the blue mass, and the veins, 

 filled with blood, is then very striking, 

 particularly in the mesentery. When the 

 arteries are well-filled, withdraw the nozzle 

 from the heart and instantly draw the thread 

 tight and knot it so as to prevent escape of 

 the injection. Then place the whole frog in 

 spirit (methylated spirit 3 parts, water I 

 part), for a few hours, after which time the 

 injection mass will be found to have set hard enough to allow of the 

 arteries being conveniently traced. 



Injection of the Veins. — The veins are much more difficult to 

 inject tlian the arteries, but if you wish to make a double injection on 

 the same specimen, colour the injection-mass with vermilion or carmine 



Fig. 23. — Sketch showing the 

 method of injecting the 

 frog's arteries. 

 «. glass " medicine-dropper '' 

 with india-ruhber cap (b) ; 

 its pointed end (dotted) is 

 passed through the cut end 

 of the ventricle (z/) into 

 the anus (c. art) ; au. the 

 auricular division ; i. 

 thread. 



