MUSCULAR SYSTEM SKELETON. 601 



Muscular System. 



The largest pectoral muscle (pectoralis major) arises from 

 the sternum and its keel, and from the clavicle ; is inserted 

 on the humerus; and depresses the wing. The. smaller but 

 longer muscle (pectoralis minor), exposed when the large 

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

 and sides of the sternum, and is continued over the shoulder 

 to its insertion on the dorsal surface of the humerus. Aris- 

 ing 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 raising 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 are continued upwards in flexor muscles over the metatarsal joint 

 to the tibia, and are flexed automatically when the leg is bent dur- 

 ing perching. Furthermore, 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 flexors of two digits. When the leg is 

 bent in sitting, the ambiens tendon is stretched, and the digits clasp the 

 branch. Thus the bird, when asleep, does not fall off its perch. 



In connection with the muscular system, we may also 

 notice that the walls of the gizzard consist of thick muscles 

 radiating around tendinous discs. Two small sterno- 

 tracheal muscles ascend from sternum to trachea. Complex 

 muscles are associated with the song-box. 



Skeleton. 



In Birds there is a marked tendency to fusion of bones, 

 as seen in the skull, vertebral column, pelvis, and limbs. 

 In the pigeon most of the bones, except those of the tail, 

 fore-arm, hand, and hind-limb, contain air spaces. 



The vertebral column is divided into five regions cer- 

 vical, thoracic, lumbar, sacral, and caudal. In the pigeon the 

 mobile neck consists of fourteen cervical vertebrae with 

 cervical ribs, short except in the last two, which have 

 them well-developed. Of the thoracic vertebrae, namely 

 those whose ribs reach the sternum, the anterior four are 

 fused to one another, while the fifth is fused to the sacral 



