432 DISSECTION OF THE PIGEON. 



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 thoracic sacs. 

 They lie 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 consists 

 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 lie 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 

 with a solution of chromic acid (2 per cent.), and the whole 

 bird then thoroughly hardened by immersion in the same 

 solution for about a week. The specimen is then to be 

 preserved in spirit. 



