THE PLEURAL SACS 341 



from one another both in size and direction. The right 

 bronchus is the wider but shorter of the two, and it is also 

 stated to be more vertical in its course (Fig. 121). Foreign 

 bodies which pass into the trachea almost invariably enter the 

 right bronchus. This route is selected, partly because the right 

 bronchus is the wider, and partly owing to the fact that the 

 bifurcation is marked in the interior of the trachea by an 

 antero-posterior ridge, which is placed slightly to the left of the 

 middle line of the trachea (Fig. 113). As the foreign body 

 descends, in most cases it impinges on the right side of this 

 ridge even when the bifurcation is placed to the right of the 

 median plane and so it is conducted into the right bronchus. 



On this account, too, the orifice of the left bronchus may 

 not be visible in bronchoscopy . It is, however, always possible 

 to observe the vibrations of the left wall of the trachea, which 

 are due to its intimate relationship to the aortic arch, and, if 

 the point of the instrument is passed downwards in close con- 

 tact with this wall, it will eventually enter the left bronchus. 



One inch from its origin from the trachea, the right bronchus 

 gives off the eparterial bronchus, which proceeds to the upper 

 lobe of the right lung. The corresponding branch on the left 

 side arises from the bronchus at a distance of 2 inches from 

 the trachea. Owing to the proximity of the point of origin of 

 the right apical bronchus to the trachea, bronchial breathing 

 is frequently heard on auscultation of the right apex and does 

 not necessarily possess any pathological significance. 



THE PLEURAL CAVITIES 



Each lung is enveloped in a serous envelope, which is 

 termed the pleural sac. The pulmonary pleura is firmly 

 adherent to the surfaces of the lung and covers the contiguous 

 surfaces of adjoining lobes. The parietal pleura lines the 

 cavity in which the lung is situated ; and, for descriptive 

 purposes, is subdivided into (i) The costal pleura, which 

 lines the inner surfaces of the ribs and intercostal spaces ; 



