THE CIRCULATION OF THE BLOOD. 



47 



paper, rolled so that the short ends lap half an inch, repre- 

 sents the aorta. The three valves are nearly semicircular 

 in shape, like the outlines in dotted lines in Fig. 5. Each 

 one is two inches wide and two and a half inches deep. 

 These valves are left free at their upper edges, which 

 should be flat on the paper, and are sewed along the 



FIG. 5. A model of the semiluuar valves. 



continuous line marked A A A A. After the valves are 

 sewed in place, roll the paper so that B comes at C and 

 sew. Make the model twice the size of Fig. 5. 



1. Hold the model in the position corresponding to the 



aorta in the body. Are the valves placed upward 

 or downward on the walls of the aorta? 



2. Pour water through the tube to imitate the flowing 



of the blood through the aorta. 



a. What is the position of the valves ? 



b. What is the effect of the valves on the stream of 

 water or blood ? 



