324 



MANUAL OF PHYSIOLOGY. 



FIG. 149. 



windpipe by approximating the extremities of the C-shaped 



springs that normally preserve its patency. 



In the bronchial tubes a large quantity of smooth muscle cells 



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



is best developed in the small tubes (Fig. 148, 6). 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 pleural cavity 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 sac. 

 3. The thorax, in which the lungs are 

 placed, is a bony framework, the dimen- 

 sions of which can be altered by the muscles 

 which close in and complete the cavity. 



The framework 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 

 Drawing of the lat- f rout by tne sternum, to which they are 

 in attached by resilient cartilaginous springs. 

 The ribs slope downwards and forwards, 

 and are more or less twisted on themselves 

 about 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 downwards in front to reach the sternum. Each suc- 



the position of gentle 

 inspiration, showing 

 the downward slope of 

 the ribs. 



