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6o SCIENCE PRIMERS. . [§ V. 



Ipart of the body ; A^. arteries to the lower part of the body : H.A. hepatic 

 artery, which supplies the liver with part of its blood ; V^. veins of the upper 

 part of the body; V^. veins of the lower part of the body; V.P. "ena 

 jportae; ./". r". hepatic vein ; V.C.I, inferior vena cava ; V.C.S. superior vena 

 cava; R A. right auricle ; R.V. right ventricle ; P. A. pulmonary artery ; 

 1 Lg. lung ; P. V. pulmonary vein ; Let. lacteals ; Ly. lymphatics ; Th.D. 

 i thoracic duct ; Al. alimentary canal; Lr. liver. The arrows indicate the 

 1 course of the blood, lymph, and chyle. The vessels which contain arterial 

 blood have dark contours, while those which carry venous blood have light 

 contours. 



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- two chambers, an upper chamber and a lower cham- 

 ber ; so that altogether we have four chambers, — two 

 upper chambers, one on each side, marked R,A, and 

 L.A,, these are called the right and left auricles ; 

 and two lower chambers, one on each side, marked 

 R, K and Z. V., these are called the right and left 

 ventricles. The right auricle, R.A., opens in the 

 direction of the arrow into the right ventricle, R. V., 

 the opening being guarded, as we shall see, by a 

 valve. The left auricle, Z.A,, opens into the left ven- 

 tricle, Z. v., the opening being likewise guarded by a 

 valve ; but you have to go quite a roundabout way to 

 get from either the right auricle or ventricle to the 

 left auricle or ventricle. Let us see how we can gel 

 round the figure. Suppose we begin with the two tubes 

 marked V.C.S. and V.C.I., the wall; of which are 

 drawn with thin lines. These both open into the right 

 auricle. They are the vena cava superior and inferior, 

 which you have just made out in the sheep's heart. 

 From the right auricle you pass easily into the right ven- 

 tricle ; thence, following the arrow, the way is straight 



* into the tube marked P. A. This is the pulmonary artery, 

 the outside of which you saw in the sheep's heart (Fig. 



' S, F.A.) Travelling along this pulmonary artery, you 

 come to the lungs, and after passing through branches 



' not represented in the figure, picking your way through 

 arteries which continually get smaller and smaller, 



