396 



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 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 formed posteriorly by the thoracic vertebrae 

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



Ext. Intercostal 



Int. Ititercostal 



Intercartilaginei 



lewifons Costaruw 

 htJntercosfat 



FIG. 183. SHOWING THE SITUATION, THE 

 POINTS OF ATTACHMENT, AND DIRECTION OF 

 THE INTERCOSTAL MUSCLES, i. The inter- 

 costales externi. 2. The intercostales in- 

 terni. 3. The intercartilaginei. 



IntJnterwstat 



FlG. 184. VIEW FROM BEHIND OF FOUR 

 DORSAL VERTEBRAE AND THREE ATTACHED 

 RlBS, SHOWING THE ATTACHMENT OF THE 

 ELEVATOR MUSCLES OF THE RIBS AND THE 

 INTERCOSTALS. (After Allen Thomson.) 



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 

 three, which are almost horizontal. 



