CIRCULATION OF THE BLOOD. 



Fig. 8. 



np % 



aorta, ( Fig.8,aa.*) ; the blood, at the 

 moment of its contraction, must 

 then either flow back into the auricle, 

 ap (o(/.) or enter the aorta. 



50. Now, around the edges of this 

 auricvh- ventricular opening, ( Fiy. 



va.) there is a sort of valve, called 

 the mitral valve, which is so arranged 

 as to rise up, and close this opening, 

 when it is pushed from below upwards. 

 From this construction, it happens, 

 that when the blood tends towards 

 fig. 9. 



entering into the auricle Co,) the mttral valce is 

 pushed up, and interrupts the communication 

 between the auricle and ventricle. 



51. Therefore, when the left ventricle con- 

 tracts, the blood finds no other outlet than the o 

 aorta, and enters this vessel which it distends ra 

 with more or less force, for its parietes, as well 

 as those of all the arteries, are very elastic. 



52. Other valves situate at the entrance of 

 the aorta, prevent the blood from returning into 



vc 



Explanation of Fig. 8. The heart opened to s'^ow the cavities in the in- 

 terior of this organ. o</, right auricle of the heart, </, right venlricle of 

 the heart, --o#, left auricle of the heart, ^, left ventricle of the heart, 

 vcs, and vci, superior and inferior venae cavae, emptying' into the right 

 auricle of the heart, ap, pulmonary artery going from the right ventricle to 

 the lungs, vp, pulmonary veins passing from the lungs to the left auricle 

 of the heart, aa, the aorta, o, l<;ft auriculo-ven'.riculur opening, o', 

 right auriculo-ventncular opening. The arrows point out the direction of 

 the flow of the blood in the different ve>sels. 



Explanation of Fig. 9. The right auricle of the heart opened to show 

 the position of the valve, , in the auriculo-ventricular opening, which, 

 during the contraction of the ventricle, (ww,) prevents the blood from enter- 

 ing the auricle, (o,) ; we observe small cords passing from the edge of this 

 valve and attaching themselves by their inferior extremities to the parietes of 

 the ventricle, (c.) Like the rest of the heart, they are fleshy, and prevent 

 the valve from turning entirely over into the auricle, when the blood, pressed 

 by the ventricle, elevates it. The aorta is al*o open to show the valves, (a') 

 which surround its entrance, and which are differently arranged from those 

 of the ventricle. 



50. Where is the mitral valve placed 'I What is the use of the mitral 

 valve ? 



51. Into what part is the blood forced by the contraction of the left ven- 

 tricle ? Why does it not go back into the auricle ? 



52. What prevents the blood from returning into the left ventricle from 

 the aorta ? Are the arteries elastic or not ? 



