46 



3. In a boiled sheep's heart, twist off the auricles, aorta 

 and pulmonary artery, and examine the auriculo-ventricular 

 and pulmonary fibrous rings. 



Clear off the visceral pericardium of the ventricles and 

 study the course of the muscle fibres. 



FIG. 14. Heart of Frog from behind. 



1. Ventricle. 4. Sinus. 



2. Auricle. 5. Great Veins. 



3. Crescent. 6. Aorta. 



4. In the longitudinal section of the heart given in the 

 Histology Class, study the various parts under a low power. 



5. Dissect the heart of a dead frog. Identify the sinus, 

 auricles, ventricle and bulbus arteriosus (fig. 14). Thrust a 

 small test-tube down the gullet to stretch it, and dissect out 

 the vagus nerve and follow its cardiac branch down to the 

 heart. 



II. Mode of Action of the Heart. 

 THE CARDIAC CYCLE. 



1 . Study the exposed heart in the frog. Pith a frog and pin 

 it out on its back on a cork plate. Open the abdomen by 

 an incision a little to one side of the middle line to avoid 

 the anterior abdominal vein, and carry the incision up to the 

 xyphisternum. Then snip through the shoulder girdle in the 

 anterior median line, taking care that the point of the scissors 

 does not injure the heart. Separate widely the two sides of 

 the girdle, pinning each back, and thus expose the heart in 

 the pericardium. Snip through the pericardium and study the 

 auricles, ventricle and bulbus as seen from the front. 



Study and describe the changes in shape which each part 

 undergoes the relative duration of each change in each part 



