THE AIR-SACS. 435 



It arises from the dorsal part of the side of the 

 keel of the sternum, and from the inner part of the 

 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 deltoid ridge. 



Owing to the course taken by its tendon, the 

 muscle elevates the wing, and so opposes 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 bach 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 ventral two- 

 thirds of the coracoid, and from the costal process 

 of the sternum. Its fibres run forwards and out- 

 wards, to be inserted into the postaxial border of 

 the head of the humerus. It aids the great pectoral 

 muscle in depressing the wing. 



III. DISSECTION OF THE AIR -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 

 respiration 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 ex- 

 pansion causes an inrush of air along the trachea, through 



