200 ANATOMY OF THE DOMESTIC POWL 



long-continued flight possible. The;^ are best developed in those 

 birds which fly most. There are four pairs of cells and one single 

 cell from which all other expansions and extensions are made. 

 These sacs are as follows: a single anterior thoracic, and, in pairs, 

 cervical, anterior diaphragmatic, posterior diaphragmatic, and 

 abdominal. 



The Anterior Thoracic Air-sac. — The anterior thoracic air-sac is 

 located above the clavicles and the interclavicular space, in the 

 cavity of the thorax. It is related superiorly with the trachea and 

 the esophagus; laterally with the lungs and the cervical air-sacs; 

 inferioriy with the. sternum, the clavicle, and the interclavicular 

 aponeurosis; posteriorly with the heart and the anterior diaphrag- 

 matic reservoir; and anteriorly with the integuments of the neck. 

 It contains the inferior larynx and the two primary bronchi, and 

 large vascular trunks from which are given off vessels suppljdng the 

 neck and the wings. 



Three prolongations, called subpectoral, subscapular, and middle, 

 or humeral, arise from the lateral walls of this air-sac. These 

 prolongations cross the walls of the thorax and pass around the 

 articulation of the shoulder. 



The subpectoral prolongation extends from the thoracic reservoir 

 by an orifice situated behind the coracoid, and passes beneath the 

 tendon of the great pectoral muscle. When the pectoraUs major 

 contracts, this contraction dilates the subjacent cell and draws into 

 it a greater quantity of air. 



The subscapular and the humeral prolongations communicate with 

 the thoracic air cell by a common opening situated behind the small 

 adductor muscle of the humerus. The subscapular prolongation, 

 after leaving this point, spreads under the scapular and the sub- 

 scapular muscle, which it separates from the ribs and corresponding 

 intercostal muscles, and extends in a longitudinal direction. 



The humeral prolongation, smaller than the subscapular, occupies 

 the axilla, and is in shape triangular. It has from its summit into 

 an infundibular fossa, an extension which enters the canal of the 

 humerus. The walls of this cell form the lining of the air space in 

 the humerus. 



The thoracic air-sac thus possesses numerous membranous folds 

 which divide its cavity. The contiguous structures which it over- 

 lies, as the trachea, the esophagus, the muscles of the inferior larjoix, 

 as well as the arteries and veins, make its outer walls irregular. 



