i88 INTRODUCTION TO GENERAL PHYSIOLOGY 



The heart is prepared thus : A frog, whose central nervous 

 system has been destroyed, is laid on its back. The heart is 

 exposed by removing the sternum (the bone in front between the 

 fore-legs), cutting the bones which unite it to the legs with strong 

 scissors. The beating heart will be seen in a 

 transparent bag (the pericardium). Open this 

 carefully and expose the heart, which has the 

 appearance of Fig. 15. By aid of a large needle, 

 fixed into a wooden handle by its point, a thread 

 is passed between B and A, taking care not to 

 injure the auricles. The needle will be more 

 handy if bent after heating in a flame, and then 

 carefully smoothed by emery paper (Fig. 16). p JG z - 

 The heart is now turned up forwards, after cutting v? is the ventr i c i ei 

 through a little fibrous band which connects it to A', the two auricles. 

 the pericardium. The thread is tied by a loose B > the commence- 



i j,i -i i j.i ment of the main 



knot around the auricles, a snip made in them as arte ries carrying 



far from the ventricle as possible, the end of the blood, expelled 



canula is inserted, and the thread tied around it. by the contraction 



By cutting through the tissues behind the heart t f 



with scissors, the heart is removed tied on the end general. 

 of the canula. The siphon is now connected up, 

 and, by sucking gently at the top of the canula, the solution flows 

 into the heart and out by the cut ends of the main arteries. -The level 

 of the solution in the canula should be 2 or 3 cm. above the heart. 

 Use first a solution containing 0.65 per cent, of sodium chloride 



P'iG. 16. Simple Aneurism Needle. 



and 0.014 per cent, of potassium chloride, in distilled water. The 

 sodium chloride should be free from calcium, if marked results are 

 to be obtained. The beats will be small and may in time dis- 

 appear. Take next a solution containing calcium ions in addition 

 to the sodium and potassium ions, namely, 0.65 per cent, sodium 

 chloride, 0.014 per cent, potassium chloride, and 0.012 per cent. 



