268 TEXT-BOOK OF PHYSIOLOGY 



developed. They lie close to the inner surfaces of the air-cells. The 

 blood is thus brought into intimate relationship with the pulmonary 

 air and the exchange of gases the excretion of carbon dioxid and 

 the absorption of oxygen for which the cardio-pulmonary vessels 

 exist, is readily accomplished. 



The pulmonary veins which return the blood to trie heart are 

 formed by the convergence and union of the small veins which emerge 



from the capillary sys- 

 tem^, The final trunks 

 thus formed, the four 

 pulmonary veins , two 

 from each lung, enter 

 the posterior wall of the 

 left auricle^SL 



The Cotlrse of the 

 Blood through the 

 Heart. There is thus 

 established a pathway 

 between the venae cavae 

 on the right side and the 

 aorta on the left side, by 

 way of the right side of 

 the heart, the cardio- 

 pulmonary vessels, and 

 the left side of the heart. 

 The venous blood 

 flowing toward the heart 

 is emptied by the supe- 

 rior and inferior venae 

 cavae into the right auri- 

 cle, from which it passes 

 through the auriculo- 

 ventricular opening into 

 the right ventricle (Fig. 

 115); thence into and 

 through the pulmonary 

 artery and its branches 

 to the pulmonary capil- 

 laries, where it is arterialized by the exchange of gases the giving 

 up of a portion of carbon dioxid to the lungs and the absorption 

 of oxygen and changed in color from bluish-red to scarlet. T' 

 arterialized blood, flowing toward the heart, is emptied by thejpul- 

 monary veins into the left auricle, from which it passes througn the 

 auriculo-ventricular opening into the left ventricle; thence into the 

 aorta and its branches to the systemic capillaries, where it is 



FIG. 115. DIAGRAM or COURSE or BLOOD THROUGH 

 THE HEART. 1,2. Superior and inferior venae 

 cavse. 3. Right auricle. 4. Right ventricle. 

 5, 5, 5. Pulmonary artery and branches. 6, 6. 

 Pulmonary veins. 7. Left auricle. 8. Left 

 ventricle. 9. Aorta. 10. Innominate artery. 

 n. Left carotid artery. 12. Left subclavian 

 artery. (After Moral and Doyon.} 



