muscLes and skeleton 737 



Muscular system. — The largest breast muscle (pec- 

 toralis major) arises from the sternum and its keel, and 

 from the clavicle, is inserted on the ventral surface of the 

 humerus, and depresses the wing. The smaller but longer 

 pectoralis minor or subclavian, exposed when the large 

 one is reflected, raises the wing. It arises from the keel 

 and sides of the sternum ; its tendon runs over the 

 shoulder (through the foramen triosseum^ which serves as a 

 pulley) to its insertion on the dorsal surface of the humerus. 

 Arising chiefly from the coracoid, but in part from the 

 sternum, and inserted on the humerus, is a small coraco- 

 brachialis, which helps a little in depressing the wing. 

 There are several yet smaller muscles. 



Interesting also is the mechanism of perching. When the bird sits 

 on its perch, the toes clasp this tightly. The flexor tendons of the 

 toes (perforati muscles) are stretched automatically when the ankle is 

 bent in perching. In some birds, an ambiens muscle, inserted on the 

 front of the pubis, is continued down the anterior side of the femur, 

 and its tendon, bending round the knee to the opposite side of the tibia, 

 is inferiorly connected with the tendon of the flexor of the second or 

 third toe, or with the third and fourth. It has nothing to do with 

 bending the first toe, and its importance has been exaggerated. The 

 bending of the toes is mainly due to the perforati muscles. 



In connection with the muscular system, it may also be noted that 

 the walls of the gizzard consist of thick muscles radiating around 

 tendinous discs. Two small sterno -tracheal muscles ascend from 

 sternum to trachea, and are apt to be confused in dissection with the 

 carotid arteries. Complex muscles are associated with the song-box. 



Skeleton. — The skeleton of birds is lightly built, with 

 much strength and surface for its weight, on the hollow 

 girder principle. The texture of the bone is often very 

 spongy, and air-sacs from the lungs may be continued into 

 many of the bones, which are then more or less completely 

 destitute of marrow in adult life. In the pigeon, most of 

 the bones, except those of the tail, forearm, hand, and 

 hind-limb, contain air-spaces. Another general character 

 is the marked tendency to fusion of bones, as seen in the 

 skull, dorsal vertebrae, sacral vertebrae, ploughshare bone, 

 carpo-metacarpus, and tarso-metatarsus. 



The vertebral column is divided into five regions — cer- 

 vical, thoracic, lumbar, sacral, and caudal. The mobile 

 neck consists of fourteen cervical vertebrae ; from the third 

 to the twelfth these bear short ribs fused to the centra and 



47 



