340 TEXT-BOOK OF PHYSIOLOGY. 



network which surrounds and embraces the air-sacs on all sides. 

 As this network is to subserve the respiratory exchange of gases 

 it lies nearer the inner than the outer surface of the lobule and 

 in close relation to the respiratory epithelium. The air and blood 

 are thus brought into intimate relationship, being separated only 

 by the respiratory epithelium and the wall of the capillary vessel. 

 The blood emerging from the capillary vessels is conducted by a 

 corresponding converging system of vessels, the pulmonary veins, out 



V5 



FIG. 163. BRONCHI AND LUNGS, POSTERIOR VIEW, i, i. Summit of lungs. 2, 2. Base 

 of lungs. 3. Trachea. 4. Right bronchus. 5. Division to upper lobe of lung. 

 6. Division to lower lobe. 7. Left bronchus. 8. Division to upper lobe. Q. 

 Division to lower lobe. 10. Left branch of pulmonary artery, n. Right branch. 

 12. Left auricle of heart. 13. Left superior pulmonary vein. 14. Left inferior 

 pulmonary vein. 15. Right superior pulmonary vein. 16. Right inferior pul- 

 monary vein. 17. Inferior vena cava. 18. Left ventricle of heart, 10. Right 

 ventricle. (Sappey.} 



of the lungs and into the left auricle of the heart. The main func- 

 tion of the pulmonary apparatus and the pulmonary division of 

 the circulatory apparatus is to afford a ready means for the exhala- 

 tion of the carbon dioxid and the absorption of oxygen. In conse- 

 quence of this exchange of gases the blood changes in color from dark 

 bluish- red to scarlet red. The relations of the heart and its vessels 

 to the lungs and bronchial tubes are shown in Fig. 163. 



The Thorax. The thorax, in which the respiratory organs and 



