So8 BIRDS. 



is continued down the anterior side of the femur, bends 

 round the knee to the opposite side of the tibia, and 

 is inferiorly connected by tendons with the flexors of the 

 digits. When the leg is bent in sitting this muscle is 

 stretched, and the digits clasp the branch. Thus the bird 

 when asleep does not fall oif 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 the sternum to the trachea, 

 and move the latter. Of importance are the complex 

 muscles 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 all the bones, except those of the tail, 

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



The vertebral column is divided into four regions- 

 cervical, thoracic, 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 

 region. The complex sacral region consists of the last 

 thoracic (with ribs), two or three lumbars, three or four 

 sacrals, and six caudals all fused. Lastly, there are about 

 six free caudals, ending in a pygostyle or ploughshare bone, 

 which represents a fusion of several vertebrae. 



When we examine one of the cervical vertebrae, we notice 

 that the anterior surface of the centrum has a complex and 

 distinctive curvature. It is concave from side to side, con- 

 vex from above downwards. Posteriorly the curvatures are 

 of course the reverse. The centra are often shortly described 

 as proccelous and cylindroidal. On the vertebra will also 

 be seen the expanded transverse processes, perforated on 

 each side by an aperture for the vertebral artery, the anterior 

 articular processes or zygapophyses, the posterior articular 

 processes, the large neural arch culminating in a neural spine. 



The ribs, borne by five vertebrae, have two heads, — a 

 capitulum articulating with a centrum, a tubercle articulating 

 with a transverse process. The ventral part of the rib 



