392 TEXT-BOOK OF PHYSIOLOGY 



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 pulmonic veins, out of the lungs and into the left auricle of the 

 heart. ' The main function of the pulmonic apparatus and the pulmonic 

 division of the circulatory apparatus is to afford a ready means for the 

 exhalation 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. 184. 



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

 associated structures are lodged, is conic in shape, though somewhat com- 



10 



13 



Vo 



FIG. 185. 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. 9. Division to lower lobe. 

 10. Left branch of pulmonic artery, n. Right branch. 12. Left auricle of heart. 13. Left 

 superior pulmonic vein. 14. Left inferior pulmonic vein. 15. Right superior pulmonic vein 

 16. Right inferior pulmonic vein. 17. Inferior vena cava. 18. Left ventrice of heart. 19. 

 Right ventricle. (Sappey.) 



pressed from before backward. Its apex is directed upward, its base down- 

 ward. The walls of the thorax are composed, first, of a bony framework 

 or skeleton and, second, of muscles and fascia. 



The bony framework is formed posteriorly by the thoracic vertebrae 

 and the posterior extremities of the ribs, laterally by the ribs, and anteriorly 

 by the costal cartilages and the sternum. The superior opening, through 

 which pass the trachea, esophagus, and blood-vessels, is oval in outline and 

 measures from side to side about 12.5 cm., and from before backward about 

 6.25 cm. The inferior opening is of large size, but irregular in its boundaries 

 from the upward inclination of the ribs and the downward projection of 

 the sternum. 



The ribs, which form a large part of the thoracic walls, constitute a series 



