THE CIRCULATION OF THE BLOOD. 39 



tion. Where does the water escape? Cut through the 

 wall of the ventricle from the base to the apex and spread 

 it open, exposing the tricuspid valve. 

 Notice : 



a. The flaps. How many are there and how are they 

 arranged ? 



b. How are they held in place ? The white cords are 

 called chordae tendinae. 



c. One end of these cords is attached to the edge of the 

 valve. To what is the other end attached? These are 

 the papillary muscles. Explain how they work. Why is 

 something of this sort necessary? 



d. Sketch. 



II. THE SEMILUNAR VALVES AT THE BASE OF THE 



PULMONARY ARTERY. 



Find where the pulmonary artery opens into the right 

 ventricle. Cut away enough of the wall of the ventricle 

 to see the opening from below. Pour water into the 

 artery and watch the action of the valves. 



Notice : 



a. How many are there ? 



b. What is their shape ? 



c. What is their arrangement? 



d. What effect did they have on the stream of water? 



e. What effect did the water have on them ? 

 /. Sketch. 



III. THE MITRAL OR BICUSPID VALVE. 



Remove the left auricle, noticing from the inside where 

 the pulmonary veins open into it. Inject water into the 



