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TEXT-BOOK OF PHYSIOLOGY. 



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- 

 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 



10 



18 



V5 



FIG. 184. 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 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 pulmonary vein. 17. Inferior vena cava. 18. Left ventricle of heart. 19. 

 Right ventricle. (Sappey.) 



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 

 of bony arches attached posteriorly to the vertebrae and anteriorly to the 

 sternum through the intermediation of their cartilages. The last two form 

 an exception. The ribs are somewhat twisted upon themselves and pursue 

 an oblique direction from above downward and forward. As a result the 

 anterior extremity lies at a lower level than the posterior. The costal 

 cartilages are directed upward and forward, with the exception of the upper 



