THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 179 



may not be able to locate all of those mentioned below, but some of them will 

 be seen. They are best studied in freshly killed specimens in which they have 

 been inflated through the trachea. 



Dorsal to the crop in the angle formed by the two halves of the furcula or 

 wishbone is situated the interclavicular air sac. Its delicate ventral wall is in 

 contact with the dorsal wall of the crop. It consists of two lobes, one on each 

 side of the median line; in the embryo these lobes are separate. Puncture the 

 interclavicular air sac and find, dorsal to it on each side, another sac, the cervical 

 air sac. 



Extend the median ventral incision in the skin to the anus. Separate the 

 skin from the underlying muscles on each side of chest and abdomen. The 

 great pectoral muscles are revealed immediately internal to the skin and occupy- 

 ing the angle between the keel and the body of the sternum. The pectoralis 

 major is the great muscle covering the entire sternum and extending to the 

 humerus. It takes origin from the keel of the sternum, the surface of the body 

 of the sternum, and the furcula which will be found imbedded in its anterior 

 border; its fibers converge toward the humerus, and passing over the shoulder 

 are inserted on the outer and dorsal surface of the humerus. The muscle should 

 be followed to its insertion. Action, depresses the wing. Now carefully cut 

 through the pectoralis major slightly to the right of the keel of the sternum 

 and along the posterior margin of the furcula. The muscle can then be deflected 

 and separates easily from the underlying pectoralis minor. The large pectoral 

 arteries and veins will probably be noticed emerging between the pectoral muscles 

 which they supply. The pectoralis minor originates from the body of the sternum 

 and converges toward the humerus. On following the muscle laterally there 

 will be found between it and the pectoralis major another air sac, the axillary 

 sac. Cut into the axillary sac. The anterior wall of this sac is in contact with 

 the coracoid bone, and laterally on looking into the sac the tuberosities of the 

 humerus will be seen. A large opening into the humerus, the pneumatic foramen, 

 is readily noticed; on probing this it will be found to lead into the interior of the 

 humerus. It is the entrance to the air space of the humerus which communi- 

 cates with the axillary air sac. The axillary air sac communicates in front with 

 the interclavicular air sac. The pectoralis minor may now be followed to its 

 insertion. It converges to a tendon which passes ventral to the posterior end 

 of the cervical air sac and beneath the shoulder to the dorsal side of the humerus 

 on which it is inserted. To see the insertion turn the bird dorsal side up and 

 dissect away the superficial muscles of the dorsal side of the shoulder. The ten- 

 don of the pectoralis muscle, owing to its mode of insertion, has a pulley-like 

 action which enables the muscle to raise the wing. Whereas in mammals all 

 of the pectoral muscles act together to adduct the fore limb, in birds the actions 

 of the pectoralis major and minor are opposed to each other, the one depressing, 



