PECTOEAL MUSCLES 389 



ventral surface of its body. Its fibres run forwards 

 and outwards, converging to be inserted into a long 

 tendon, which passes through the foramen triosseum, 

 formed by the scapula, coracoid, and clavicle, and 

 reaches the back of the shoulder -joint, where it is 

 inserted into the dorsal surface of the humerus, 

 near the greater tuberosity. 



Owing to the course taken by its tendon, the 

 muscle elevates the wing, and so antagonises the 

 great pectoral muscle. 



To see the insertion of the tendon of the second pectoral 

 muscle, remove or push aside the deltoid muscle, which covers 

 the back of the shoulder -joint. Test the action of the muscle 

 by pulling the tendon. 



3. The third pectoral muscle is a small triangular muscle, 

 arising from the outer border of the posterior two- 

 thirds of the coracoid, and from the costal process of 

 the sternum. Its fibres run forwards and outwards, 

 to be inserted into the postaxial border of the head 

 of the humerus. It aids the great pectoral muscle 

 in elevating the wing. 



III. DISSECTION OF THE AIK-SACS. 



The air- sacs are large but very thin- walled sacs, lying 

 mainly along the sides and dorsal surface of the body-cavity. 

 They communicate, as already noticed, with the bronchi, and 

 they play a very important part in the mechanism of respira- 

 tion of the bird. 



Expiration is effected by the thoracic and abdominal 

 muscles, which compress the thorax and abdomen, driving 

 the air from the air- sacs, through the lungs and trachea. 



Inspiration is effected by the elastic expansion of the 

 thorax and abdomen on relaxation of the muscles : this 

 expansion causes an inrush of air along the trachea, through 

 the lungs and into the air-sacs, which are thus filled with 

 fresh air. 



