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CIRCULATION.] . - rHYSIOLOGY, 75 



■ To return to the inside of the left ventricle. Up 

 at the top of the ventiicle, close to the opening from 

 the auricle, there is one other opening, and only one. 

 If you put your finger into this, you will find that it 

 leads into a tube which first of all dips under or be- 

 hind the pulmonary artery and then comes up and to 

 the front again. This tube is what you already know 

 as the aorta. If you slit it up from the ventricle (and 

 to do this you must cut through the pulmonary artery), 

 you will find that on the left side, as on the right, the 

 red fleshy wall of the ventrick suddenly changes into 

 the yellow firm wall of the artery, and that just at this 

 line there are three semilunar valves exactly like those 

 in the pulmonary artery. ■ ' * '^ , 



On the left side of the heart, then, we have f 

 also two valves, the mitral between the 

 auricle and the ventricle, and the semilunar 

 between the ventricle and the aorta. These \ 

 let the blood pass one way and not the other. : 

 You can easily drive fluid from the pul- 

 monary veins through auricle and ventricle 

 into the aorta, but you cannot send it back 

 the other way from the aorta. 



These then are the reasons why the blood will only 

 pass one way, the way I said it did. There are sets | 

 of valves opening one way and shutting the other. | 

 These valves are the tricuspid between the right 

 auricle and right ventricle, the pulmonary semilunar 

 valves between the right ventricle and the pulmonary 

 artery, the mitral valve between tlie left auricle and the | 

 left ventricle, the aortic semilunar valves between the 

 left ventricle and the aorta, and the valves '.yhich are 

 scattered among the veins of the body. Of these by 



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