446 AVES. 



thorax, and its inner walls are in contact with the pericardium, 

 except anteriorly and dorsally where it is adlierent to its fellow 

 (forming part of the median septum). 



The second entobronchium gives off a wide branch which passes 

 to the interclavicular sac and is joined just where it opens into 

 this sac by a branch from the first entobronchium. The pos- 

 terior part of the interclavicular air-sac lies on, and its dorsal wall 

 is united with, the ventral surface of the anterior part of the lung ; 

 this part overlaps the anterior part of the anterior thoracic. The 

 anterior part is united with its fellow and surrounds the hinder 

 end of the trachea between the two clavicles ; it gives off lateral 

 diverticula one of which passes outwards behind the coracoid 

 and dilates into a large axillary chamber near the shoulder joint 

 which communicates with the air-cavities in the humerus. 



The first entobronchium is directed forwards and, after giving 

 off the tube to the interclavicular sac already mentioned, is 

 continued to the anterior end of the lung, where it opens into 

 the small cervical sac, which lies in front of the lungs at the base 

 of the neck and dorsal to the anterior part of the interclavicular 

 sac. It lies between the lorigus colli muscle with the vertebral 

 column dorsally and the oesophagus and bronchus ventrally, 

 and is widely separated from its fellow. In many birds the cer- 

 vical air sacs are continued up the neck close to the ventral sides 

 of the vertebrae communicating with the air-cavities in these 

 structures and in front with the air-cavities in the bones of the 

 upper beak. 



The communications (ostia) on the surface of the lungs by 

 which the secondary bronchi communicate with the air-sacs are 

 for the most part well-marked openings. The ostia of the 

 abdominal sacs are at the postero-external angle of the lung ; 

 those of the posterior thoracic just anterior and external to the 

 last ; those of the anterior thoracic are nearer the middle line 

 not far behind the entrance of the main bronchus into the lung ; 

 those of the interclavicular are just anterior to the entrance of 

 the main bronchus, and the cervical air-sacs are given off from 

 the anterior end of the lung. 



The ventral surface of the lungs is invested by a thin fibrous 

 membrane called the 'pulmonary aponeurosis which is perforated 

 by the ostia of the air-sacs. This membrane is continuous in the 

 middle line with a median vertical septum which separates the 



