62 ACTION OF THE II K ART. 



valves of the aorta into the left ventricle ; it passes back of 

 one flap of the mitral valve. 



10. Cut open the left ventricle. Note the strong muscu- 

 lar columns, the strong papillary muscles ; the mitral valve, 

 though ending in two main flaps below, is continuous at the 

 top. Compare the walls of the right with those of the left 

 ventricle. Why this difference ? Note the partition between 

 the ventricles. Is there any direct communication between 

 the right and left halves of the heart ? 



11. Slit open the aorta between two of the semilunar 

 valves, and study the valves more closely. In the middle of 

 the free border of each valve note the little thickened point, 

 the Corpus Arantii. When the valves close, these three little 

 points fill up a small three-cornered opening that would other- 

 wise be left between the valves. Again examine the cardiac 

 arteries. 



12. In another heart, carefully cut around the base of the 

 pulmonary artery, tie its outer end tightly over the end of a 

 glass tube or spool, and show the action of the semilunar 

 valves, by blowing suddenly and forcibly into the tube. To 

 keep the glass tube from slipping out, slip an inch of thick 

 rubber tubing on the end of the glass tube, so that the rubber 

 tube is even with the end of the glass tube. The valves 

 work better when moist and flexible ; therefore keep the 

 preparation standing in a jar of water until it is to be used. 

 Slit open the artery, and study the valves. 



13. Longitudinal and cross sections of a frozen heart are 

 instructive. 



Demonstration of the Action of the Heart. Get the 

 heart and lungs entire. Dissect out the aorta as before. Clear 

 the pulmonary artery, and cut off both branches close to the 

 lungs. Carefully trim away the pericardium, and clean the 

 precaval and postcaval veins. Turn the heart back, and find 



