LUNGS 



385 



FIG. 291. LEFT LUNG OF THE DUCK, in situ. (From a drawing by H. Strasser.) 

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

 externally the pulmonary artery. 



an, opening of the main bronchus into the abdominal sac ; b, opening of the outer 

 lateral branch of the mesobronchium into the posterior thoracic air-sac ; 

 6 l , second ostium of the latter, more towards the middle line (present in 

 Passeres) ; Br. Ws, thoracic vertebrae ; /, first entobronchium, and c, its 

 ostium communicating with the cervical air-sac ; i, a, e, its internal, 

 anterior, and external branches ; He, Hi, 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 for the anterior thoracic 

 air-sac; IV, fourth entobronchium; m.l.c, longus colli muscle; JV, kidney; 

 Oe, oesophagus ; stv, stv, sections of ribs which are connected with the 

 sternum ; Tr, trachea ; v, v, ends of free vertebral ribs. The boundary of 

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

 costopulmonary muscles are shown extending from it to the ribs. 



The air-sacs arise from the embryonic pulmonary vesicles as 

 delicate-walled hollow processes, lined by pavement epithelium : 

 these grow rapidly, and soon exceed the lung proper in size, 

 extending amongst the viscera. Moreover, they are not confined 

 to the body-cavity, but in numerous places extend beyond it, passing 



c c 



