62 THE STRUCTURE OF THE FOWL 



are composed of a serous membrane externally 

 with a mucous membrane within. 



A single clavicular sac is placed between 

 and behind the two limbs of the furcula, and 

 is continued on each side as an axillary sac. 

 The air contained within the clavicular and 

 axillary sacs is obtained through two openings 

 at the cranial end of each lung and is trans- 

 mitted to the interior of the sternum, sternal 

 ribs, shoulder-girdle, and humerus. 



Enclosing the abdominal organs and lying 

 against the walls of the abdomen are the large 

 abdominal sacs, right and left, into which the 

 mesobronchi open. From them air enters 

 the hollows of the sacrum, hip-bone, and 

 femur. 



Lying partly in the neck and partly dorsal 

 to the clavicular sac are the two cervical sacs 

 which supply air to the cervical and thoracic 

 vertebrae and the vertebral ribs. Within the 

 chest are two thoracic sacs — cranial and caudal 

 — on each side. These stretch from the clav- 

 icular to the abdominal sacs, and, unlike the 

 other air-sacs, do not communicate with the 

 interior of bones. 



The air-sacs appear in the chick during the 

 first seven or eight days of incubation as 



