CONSTRUCTION OF THORAX. 



329 



FIG. 149. 



exist, for the most part being arranged as a circular coat, which 

 is best developed in the small tubes (Fig. 148, 5). As we pass 

 from the large to the smaller bronchi the walls become thinner 

 and less rigid, and the cartilaginous plates 

 and fibrous tissue gradually diminish, while, 

 on the other hand, the muscular and elastic 

 elements become relatively more abundant. 



The external surface of the lungs is com- 

 pletely invested by a serous membrane 

 the pleura, which is reflected to the wall of 

 the thorax from the roots of the lungs, and 

 completely lines the cavities in which they 

 lie. Thus the lungs are only attached to 

 the thorax where the air passages and great 

 vessels enter, the rest of their surface being 

 able to move over the inner surface of the 

 thorax, and to retract from the chest wall 

 if air be admitted into the pleural sack. 



3. The thorax, in which the lungs are 

 placed, is a bony frame-work, the dimen- 

 sions of which can be altered by the muscles 

 which close in and complete the cavity. 



The frame-work is a rounded, blunt cone 

 composed of a set of bony hoops the ribs, 

 attached by joints to a bent pliable pillar 

 the vertebral column, and held together in 

 front by the sternum, to which they are Drawing of the lateral 

 attached by resilient cartilaginous springs, view of Thorax in the po- 



. .11 i /> IT sition of gentle inspiration, 



The ribs slope downward and forward, and 8howing the downward 

 are more or less twisted on themselves about slope of the ribs, 

 the middle of the shaft. 



The first pair of ribs, which encircles the apex of the thoracic 

 cone, forms part of a short, flattened hoop. It slopes downward 

 in front to reach the sternum. Each succeeding rib from above 

 downward, increases in the amount of its slope downward and 

 forward, and in the obliquity of its shaft. 



The floor of the thorax is formed by a dome-shaped muscle 

 28 



