THE AXIAL SKELETON
This strictly comprises the vertebral column, ribs, and sternum, but the pelvis may be included because it is firmly attached to the synsacrum formed of fused lumbar, sacral, and caudal vertebrae (Figure 37-9, B).
Division of the vertebral column into its regions is made difficult by the extensive fusion and the uncertain location of the junction between the cervical and thoracic elements.
The number of cervical vertebrae varies with the length of the neck. Small birds may have only 8, while swans have as many as 25; in the chicken the number ranges from 14 to 17. The atlas (Figure 37-9/5) is a small ring that articulates by a depression in its ventral arch with the single occipital condyle. Caudally, this arch has a facet for the dens of the axis. Except for the presence of the dens and short cranial articular processes, the axis differs little from the remaining cervical vertebrae, which are uniformly cylindrical and have prominent articular processes and rudimentary cau- dally directed (cervical) ribs.
The number of the thoracic vertebrae ranges from 3 to 10; 7 in a chicken, carries complete ribs for connection with the sternum. Many species, including chickens, raptors, and pigeons, have the first three to five thoracic vertebrae fused into a single bone (notarium; Figure 37-9/13), which provides a rigid beam. This is followed by a single free thoracic vertebra, the only mobile vertebra of the trunk. This vertebra articulates cranially and caudally by synovial joints in which both the articular processes and the bodies participate. It is the weak link in the column; its cranial end may be displaced ventrally, impinging on the spinal cord (kinky back in broilers). The last one or two thoracic vertebrae fuse with the lumbar, sacral, and first caudal vertebrae to form the synsacrum (Figure 37-9/15). The synsacrum and the notarium render the dorsal part of the trunk rigid; this rigidity is extended laterally and caudally by the fusion of the synsacrum with the long hip bones.
The synsacrum is followed by five or six free caudal vertebrae that allow movement to the tail. The most caudal segment (pygostyle; Figure 37-9/17) consists of several fused rudiments and gives support to the flight feathers of the tail.As in mammals, the bony pelvis consists of right and left hip bones and the (syn)sacrum. It is deeply concave ventrally and relatively long and braces as much as half the trunk, which is an arrangement thought to be related to the bipedal posture. The broad dorsal and lateral surfaces of the hip bones are formed by the ilium and ischium, respectively (Figure 37-9/18,19). The pubis is a thin rod attached to the ventral border of the ischium (Figure 37-9/20). Ilium and ischium join to form the perforated acetabulum. Caudodorsal to this, a blunt process (antitrochanter) articulates with the trochanter of the femur and limits abduction. The hip bones do not meet in a ventral symphysis; the wide clearance favors the passage of the egg. This is not the case in the ostrich and rhea, which have a pubic symphysis that may be an adaptation to support the heavy mass of viscera.
Five or six pairs of ribs connect the extensive sternum to the thoracic vertebrae. Each complete rib consists of dorsal (vertebral) and ventral (sternal) parts that meet at a cartilaginous joint. The vertebral rib corresponds to the osseous, the sternal rib to the cartilaginous part of the mammalian rib. Most vertebral ribs present a caudodorsally directed (uncinate) process that overlaps the next rib. These processes give attachment to muscles and ligaments and strengthen the thoracic wall. Floating (vertebral) ribs from the last few cervical vertebrae precede the complete ribs.
The sternum is a large unsegmented bone, which with its processes forms a considerable part of the ventral body wall (Figure 37-9, A). It gives attachment to the large flight muscles (see further on). It has a prominent keel (carina) in many good fliers. In other species a lower keel is compensated by greater sternal width.
A keel is altogether lacking in the large flightless ratites (ostrich and rhea). The sternum of the chicken is relatively long and narrow, and although the chicken is a poor flier, it has a deep keel (Figure 37-9/28). The subcutaneous position of the keel is ideal for bone marrow sampling in chickens or large cage birds but exposes it to injury when perching (twisted or bruised keels are an important factor in grading poultry). The manubrium (Figure 37-9/26), a median process on the cranial end of the sternum, is flanked by large facets that receive the massive coracoid bones from above. Long processes, cranial and caudal to the articulations with the sternal ribs, enlarge the support provided to the lateral and ventral body wall. Pneumatic foramina on the concave dorsal surface of the sternum connect with the clavicular air sac. The caudal end of the sternum is cartilaginous in the young but later ossifies; its flexibility is thus an indicator of age.