SKELETON. 599 



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, and is continued 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 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 stretched automatically when the leg is bent in perching. 

 Furthermore, an ambiens muscle, inserted on the front of the pubis, 

 is continued down the anterior side of the femur, and its tendon bend- 

 ing round the knee to the opposite side of the tibia, is interiorly con- 

 nected 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. It is only in some 

 birds, however, that the ambiens muscle is present. 



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 vocal cords 

 in 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 usually 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 vertebra, 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 

 transverse processes; the thirteenth and fourteenth have 



