THE HEART AND THE BLOOD VESSELS 



147 



Almost before this aorta emerges from the base of the 

 heart, it gives off two small arteries which supply blood to 

 the heart muscle itself. It may seem strange that the heart 

 with the great stream of blood flowing through it needs 

 special arteries to supply it with blood. But the blood 

 flowing through the heart does not nourish it any more than 

 the sap running up the tree nourishes the outer layers of the 



Vein 



leucocyte 

 Erythrocyte 



bark. Hence the heart, 

 which works more con- 

 stantly than any other 

 organ, needs its own 

 blood supply, which 

 is received through 

 these coronary arteries. 

 The aorta goes up- 

 ward a couple of 

 inches, bends toward 

 the left, and then turns 

 downward through the 

 thorax and abdomen, 

 to the lower part of 

 the body, as shown in 

 Figure 78. Before 

 turning downward it 

 gives off branches, of 

 which the carotids pass 

 to the head on each 

 side (Fig. 78); while 

 others, the subclavians, 

 pass to each arm, thus supplying the upper extremities 

 with blood. The right carotid and right subclavian leave 

 the aorta as one trunk that soon divides. The main artery 

 (aorta) descends through the abdomen, side branches furnish 

 blood to the intestine and other organs in the abdomen, and 

 finally the aorta divides, one branch extending into each leg. 



Capillortcs 



FIG. 79. THE TERMINUS OP AN ARTERY 



Showing its connection with a vein through the 

 capillaries. In the upper capillaries are shown 

 blood corpuscles flowing through them. Some 

 leucocytes are shown making their way through 

 the capillary walls, and others quite outside of the 

 blood vessels. 



