202 ANATOMY OF THE DOMESTIC FOWL 



extension is given off, which enters the second vertebral segment at 

 the antero-lateral part; from this point it passes back, forming a 

 new air-sac between the second and third ribs. It now passes in 

 the same manner into the third vertebra and extends through the 

 third intercostal sac, and so on till the last dorsal vertebra has been 

 served. At the same time that these sacs receive the air from the 

 vertebrae preceding them, and transmit it to those which follow, 

 they communicate it to all the vertebral ribs. The aerial currents 

 which leave the cervical air-sacs do not communicate with those 

 of the cranium. Experiments show that the cranial bones have 

 apparently no communication with the respiratory apparatus. 



The Anterior Diaphragmatic Air -sac. The two anterior dia- 

 phragmatic or supero-posterior air-sacs are related with the lungs 

 anteriorly, and with the abdominal viscera posteriorly. Anteriorly 

 also is the thoracic air-sac, posteriorly are the posterior diaphrag- 

 matic air-sacs, and laterally the ribs and the intercostal muscles 

 and internally is the esophagus. The lungs communicate with these 

 air-sacs through circular openings from the great posterior diaphrag- 

 matic bronchus and frequently by a second opening from this same 

 tube. These are the only sacs which receive air from the lungs 

 through two openings. 



The Posterior Diaphragmatic Air-sac. The two posterior dia- 

 phragmatic, or infero-posterior air-sacs are oval in shape and located 

 between the thoracic and the abdominal cavity. They are related 

 anteriorly with the anterior diaphragmatic air-sacs. These two 

 sacs form a vertical transverse partition. The posterior diaphrag- 

 matic air-sacs are related posteriorly with the abdominal air-sacs 

 from which they are separated by the diaphragm. They are 

 related below with the lateral parts of the sternum and the sternal 

 ribs, and externally with the ribs and the intercostal muscles. 

 These air-sacs communicate with the lungs through openings located 

 in the middle part of the external border of the lung, into the 

 extremity of voluminous bronchial tubes which follow the direction 

 of the largest air tubes. 



The Abdominal Air-sacs. The two abdominal air-sacs located 

 on each side of the abdominal cavity, when inflated with air, form 

 enormous bladder-like structures. They are related laterally with 

 the abdominal wall and internally with the abdominal viscera. The 

 anterior extremities are in communication with the mesobronchi 

 and are somewhat inflected to pass under the fibrous arches extend- 



