LUNGS. 



261 



FIG. 210. LEFT LUNG OF THE DUCK, in situ. (From an original drawing by 



H. Strasser.) 



The main bronchus is cut open ; internally to it lies the pulmonary vein, and 

 externally the pulmonary artery. 



Oe, oesophagus ; m.l.c, muse, longus colli ; BrWs, thoracic vertebrae; v, v, ends of 

 free vertebral ribs; stv, stv, sections of ribs which are connected with the 

 sternum ; N, kidney ; Tr, trachea ; /, first entobronchium, and c, its aperture 

 of communication (ostium) with the pre bronchial air-sac ; i, Q,, e, its internal, 

 anterior, and external branches ; Hi, He, internal and external branch of the 

 second entobronchium : the end of He opens' into the sub-bronchial sac at d ; 

 ///, third entobronchium, with the aperture e for the anterior intermediate air- 

 sac ; IV, fourth entobronchium ; au, opening of the main bronchus into the 

 a'bdominal sac ; b, opening of the outer lateral branch of the mesobronchium 

 into the posterior intermediate sac ; b', second ostium of the latter, more 

 towards the middle line (present in Passeres). The boundary of the pulmonary 

 aponeurosis is seen along the outer edge of the lung, and the costo-pulmonary 

 muscles are shown extending to it from the ribs. 



unpaired, the sac of either side fusing with its fellow to form an interclavi- 

 cular chamber, bounded by the furcula ; 1 it communicates with neighbouring 



1 In some Birds (e.g. Rhea, Vulture, Adjutant) a median septum is present 

 separating the two sub-bronchial sacs. 



