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ELEMENTARY BIOLOGY 



216. The heart. In birds and mammals the heart is a double 

 muscular organ, the right and left halves being quite distinct 

 from each other, in the sense that blood cannot pass directly 

 from one side to the other. Each half of the heart consists 

 of an upper receiving chamber and a lower pumping chamber. 

 The left heart is somewhat larger and stronger than the right 

 heart. Its ventricle, or pumping chamber, closes up, or contracts, 



at fairly regular intervals, 

 forcing the contained blood 

 into the largest artery of 

 the body (the aorta), by 

 the branches of which it is 

 carried on to the various 

 organs and tissues. 



The anricle, or receiv- 

 ing chamber, of the left 

 heart is connected with 

 a large vein that brings 

 blood gathered from the 

 capillaries of the lungs. 



Fig. 69. Diagram of the human heart The receiving chamber 



opens directly into the left 

 ventricle, by means of an 

 opening which is guarded 

 by a set of flaps that pre- 

 vent the blood from flowing 

 back when the pumping 

 chamber contracts. Another set of valves prevents the blood in 

 the aorta from flowing back into the ventricle when the latter 

 expands again. The left heart thus pumps blood received from 

 the capillaries of the lungs to arteries all over the body. 



The right heart receives blood from two large veins con- 

 nected with its auricle, or receiving chamber (Fig. 69), and 

 passes it into the ventricle, or pumping chamber. The auricle 

 and ventricle on the right side are also connected with each 



aa, receiving chambers, or auricles ; l>l', pumping 

 chambers, or ventricles ; cc, main veins, bringing 

 blood to the heart ; dd, main arteries, carrying 

 blood away from the heart ; ff, valves, prevent- 

 ing back-flow from arteries to ventricles ; be- 

 tween a and b, valves preventing back-flow from 

 ventricles to auricles 



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