Mammalia. 139 



7. Examine the left auricle, and find where the pulmonary 

 veins enter it. Cut away the lobe of the left auricle ; examine 

 its inner surface, and find the openings of the pulmonary veins. 

 Hold under a faucet, and prove the action of the mitral valve, 

 between the left auricle and the left ventricle. Insert the nozzle 

 of the faucet between the valves, and again turn on the water. 

 Where does it escape ? Cut off the aorta half an inch from its 

 base, and repeat the last experiment with the water, meanwhile 

 closely watching the semilunar valves of the aorta. 



8. Above the pockets of the semilunar valves look for the 

 openings of the cardiac (coronary) arteries, which supply the 

 walls of the heart. Probe them. How many are there ? 



9. Pass the handle of the scalpel between the semilunar 

 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 muscular 

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

 ending in two main flaps below, is continuous at the top. The 

 valves between the auricles and ventricles are sometimes called 

 the auriculo-ventricular valves. This may be shortened to " aur- 

 vent " valves, and will be easily remembered, as the parts of the 

 word indicate the two cavities between which the valves lie. 

 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 otherwise be left 

 between the valves. These valves are sometimes called the 

 ventriculo-arterial, or, for short, the " vent-art " valves, as they 

 lie between the ventricles and the arteries. Again examine 

 the cardiac arteries. 



