THE AIR-SACS. 437 



2. The posterior thoracic air-sacs are a pair of rather 



smaller sacs lying along the outer sides of the 

 anterior ends of the abdominal sacs. They lie 

 mainly posterior to the lungs, but overlap the 

 ventral surfaces of their hinder ends. - 



Each communicates with the outer and posterior 

 angle of the corresponding lung. 



3. The anterior thoracic air-sacs cover the ventral surfaces 



of the anterior two-thirds of the lungs, and extend 

 back so as to overlap the posterior thoiacic sacs. 

 They he at the sides of the thorax, and are in exten- 

 sive contact with the ribs and with the pericardium. 

 Each communicates with the ventral surface of 

 the lung, rather in front of the middle of its length, 

 and not far from the median plane of the body. 



4. The interclavicular air-sac is a median sac formed by 



the fusion of two originally distinct ones. It con- 

 sists of (1) a median portion, which surrounds the 

 hinder end of the trachea, and can easily be seen 

 in the angle between the two clavicles ; and (2) 

 lateral diverticula, one of which runs forwards 

 along each side of the trachea, while others pass 

 outwards, beneath the coracoid, and open into 

 large axillary air-sacs in the neighbourhood of 

 the shoulder- joints, which communicate with the 

 cavities of the humeri. 



The interclavicular air-sac opens into the ventral 

 surface of each lung, immediately in front of the 

 point of entrance of the main bronchus. 



5. The cervical air-sacs are much smaller, and he in front 



of the lungs at the base of the neck, alongside the 

 vertebral column. 



They communicate with the anterior angles or 

 apices of the lungs. 



The position, extent, and relations of the air-sacs can be 

 best made out in a bird in which they have been distended 



