All of the coccygeal vertebræ except the first are freely movable, thus allowing the tail to be used as a rudder during flight. The lateral spines are long and well developed, and the superior spines are bifurcated, thus giving increased surface for muscular attachment. The first coccygeal segment is fused with the last lumbo-sacral vertebra. THE THORAX The dorsal vertebræ superiorly, the ribs laterally, and the sternum, or breast-bone inferiorly, form the skeleton of a large cavity called the thorax. The dorsal vertebræ have been described. =The Ribs= (Fig. 4, No. 32; Fig. 10, _A_ and _B_). _Location._—The ribs form the lateral bony wall of the thorax, articulating superiorly with the dorsal vertebræ. _Description._—The ribs are arranged in order of length, the ultimate rib being the longest. From anterior to posterior, they approach more nearly a horizontal position. The ribs are divided into the true and the false. The _true ribs_ articulate with the sternum. The _false ribs_ do not touch the sternum; they are floating. The true ribs are composed of two parts, a vertebral, or dorsal, and a sternal, or ventral. The vertebral part, or _extremitas vertebralis_, is provided with an articular head, or _caput costæ_; a neck, or _collum costæ_; and an articular tubercle, or _tuberculum costæ_. The head and tubercle articulate with the dorsal vertebra. Below the head of each rib is a _pneumatic foramen_. The distal extremity of the dorsal section of the rib articulates with the proximal end of the sternal section by a diarthrodial articulation. The articulations of the true ribs with the sternum is diarthrodial and each articulate by two small ridges with a double sternal facet. The first, the second, and the seventh ribs are floating, or false ribs. The first rib articulates with the quadrate part of the last cervical vertebra and also with the first dorsal vertebra. The seventh, the last rib, articulates with the under side of the anterior alar part of the ilium. This rib is situated similarly to the true ribs; but, instead of articulating directly with the sternum, the lower end lies against the sternal segment of the rib just anterior to it. From the posterior edge of the second, the third, the fourth, and the fifth ribs, and near the middle of the dorsal segment, are flat, uncinate, bony processes which project upward and backward, overlying in each case, the succeeding rib and giving greater surface for muscular attachments and greater stability to the thorax. =The Sternum= (Fig. 4, No. 40; Fig. 10, No. _A_, 9). _Location._—The sternum, or breast-bone forms the inferior portion, or floor of the thoracic cavity.