Page 537 - Anatomy and Physiology of Farm Animals, 8th Edition

522 / Anatomy and Physiology of Farm Animals

wing. Additionally, all birds, even relatively The last few caudal vertebrae are fused
into the pygostyle, which provides a solid
poor flyers, have skeletal, cardiopulmo-
VetBooks.ir nary, and muscular modifications that mass for stability of the tail feathers.
Cervical vertebrae are increased in num-
reflect their common ancestry in a flighted
progenitor. The body cavity has no muscu- ber relative to mammals, accounting for
lar diaphragm, being instead incompletely the remarkable flexibility of the avian neck;
partitioned into a number of compart- chickens, turkeys, and ducks have 14 and
ments by connective tissue sheets. These geese 17 cervical vertebrae (Fig. 30‐5).
are typically perforated by a number of Each thoracic limb possesses a bony
natural apertures that allow communica- attachment to the axial skeleton (in con-
tion between the compartments and with trast to the muscular synsarcosis of the
peripheral air sacs. mammalian thoracic limb). This shoulder
Bones of birds are higher in mineral girdle comprises the scapula, the coracoid
concentration than are mammals. Calcium (which is a bridge between the shoulder
turnover in egg‐laying birds is extraordi- joint and the sternum), and the clavicle.
nary and mineral loss from bone (osteopo- The right and left clavicles in birds are
rosis) is not uncommon. The mechanisms fused into a single bone, commonly called
regulating this process are covered later in the wishbone but in anatomical terms is
this chapter. The medullary cavities of called the furcula. The furcula acts as a
many long bones and some flat bones are bony strut to maintain distance between
variably displaced by air‐filled spaces that the two shoulders.
make the bones lightweight for their size In the antebrachium, the radius is
and strength. Bones modified in this way considerably smaller than the ulna. These
are called pneumatic bones and are found bones articulate with the two bones of the
most often in flighted birds. Because of carpus. The distal row of carpal bones have
the role of the thoracic limb as a wing, the become fused with the proximal end of
thoracic girdle is extremely well devel- the metacarpals, and so these long bones
oped, providing rigid scaffolding for the of the metacarpal region are known as
muscles of flight. The pelvic limbs are carpometacarpal bones, of which there
very strong for the size of the animal and are three. There are three digits. The first is
brought craniad toward the body’s center often called the pollux and like the “thumb”
of gravity (Fig. 30‐4). Both of these char- of mammals possesses two phalanges.
acteristics reflect the fact that the pelvic Digit two is robust, with two well‐devel-
limbs in birds have assumed all the oped phalanges and one vestigial phalanx.
responsibility of supporting the body The third digit is very small and comprises
weight when the bird is not in flight. only a single phalanx.
Overall, the skeleton shows a tendency There is considerable diversity in the
toward fusion between small bones that appearance of the pelvis in birds, but like
in mammals constitute complex joints the mammalian pelvis, three bones (ilium,
(e.g., carpus and tarsus). ischium, and pubis) contribute to its con-
struction. Like the thoracic limb, the pelvic
limb shows a reduction in the number of
Skeleton and Bone individual bones of the tarsus. Fusion of
the proximal row of tarsal bones with the
The avian vertebral column comprises tibia creates the tibiotarsus. The fibula is
vertebrae divided into cervical, thoracic, fused with the tibiotarsus in its distal half.
lumbar, sacral, and caudal divisions as in The distal tarsal bones are fused to a single
mammals, but vertebrae in the thoracic weight‐bearing metatarsal bone; this is
region are partially fused into the notar- therefore correctly referred to as the tarso-
ium, and in the lumbosacral region 14 to metatarsus in birds. Poultry possess four
15 vertebrae are fused into the synsacrum. digits; digit one projects in a medioplantar