HEART AND LUNGS OF MAMMAL. 141 



they arranged ? How are they held in place ? How 

 are they acted upon, and how do they act ? 



6. Find the connection between the right ventricle and 

 the pulmonary artery. Cut away the wall of the 

 right ventricle so that the beginning of the pulmo- 

 nary artery may be seen from below. Hold the heart 

 up by the pulmonary artery and force a stream of 

 water through the pulmonary artery toward the 

 heart. Look from beneath to see the filling and 

 bulging out of the valves at the beginning of the 

 artery. Note the number, shape, and arrangement of 

 these valves. What is the effect of the stream of 

 water on them, and what is their effect on the stream 

 of water? Slit the artery and examine the valves 

 from above. These are the semilunar valves. 



7. Examine the left auricle to find where the pulmonary 

 veins enter it ; cut the auricle away from the ventri- 

 cle and examine it from the inside to see the openings 

 of the pulmonary veins. Pour water into this ventri- 

 cle as with the right. Compare the valves of this 

 and the right side of the heart. 



Cut off the aorta near the heart ; watch the open- 

 ing of the aorta when water is poured into the ven- 

 tricle, to see the action of the valves. In the pockets 

 of the valves of the aorta look for the openings of 

 the arteries which supply the walls of the heart with 

 blood. 



8. Cut open the left ventricle and compare its walls 

 with those of the right ventricle. Why are they dif- 

 ferent? Note the partition between the ventricles ; is 

 there any direct communication between the right 

 and left halves of the heart ? 



