THE HEART 



41 



The shorted possible course which any particle of the 

 blood can take in order to pass from one side of the heart 

 to the other, is to leave the aorta by one of the coronary 

 arteries, and return to the right auricle by the coronary 

 vein. And in order to pass through the greatest possible 

 number of capillaries and return to the point from which 

 it started, a particle of blood must leave the heart by the 

 aorta and traverse the arteries which supply the aliment- 

 ary canal, spleen or pancreas. It then enteres Istly, the 

 capillaries of one of these organs ; 2ndiy, the capillaries 



Fig. 13.— Transverse Section of the Chest, with the Heart and 

 Lungs in Place. (A little diagrammatic.) 



D. V. dorsal vertebra, or joint of the backbone ; Ao, Ao'. aorta, the top 

 of its arch being cut away in this section ; 8. C. superior vena cava ; 

 P. A. pulmonary artery, divided into a branch for each lung ; L.P. R.P. 

 left and right pulmonary veins; 5)-. bronchi ; R.L. L.L. right and left 

 lungs ; (E. the gullet or wsophagus ; p. outer bag of pericardium ; pi. the 

 two layers of pleura ; v. azygos vein. 



of the liver ; and, Srdly, after passing tlu-ough the right 

 side of the heart, the capillaries of the lungs, from which 

 it returns to the left side and eventually to the aorta. 



4. The Heart.— The heart (Figs. 12 and 14), to which 

 all the vessels in tlie budy have now been directly or 



