Skeleton and Bone

The avian vertebral column comprises verte­brae divided into cervical, thoracic, lumbar, sacral, and caudal divisions as in mammals, but vertebrae in the thoracic region are partially fused into the notarium, and in the lumbosa­cral region 14 to 15 vertebrae are fused into the synsacrum.

Each thoracic limb possesses a bony attach­ment to the axial skeleton (in contrast to the muscular synsarcosis of the mammalian tho­racic limb). This shoulder girdle comprises the scapula, the coracoid (which is a bridge between the shoulder joint and the sternum), and the clavicle. The right and left clavicles in birds are fused into a single bone, commonly called the wishbone but in anatomical terms is called the furcula. The furcula acts as a bony strut to maintain distance between the two shoulders.

Figure 30-2. The skeleton of the chicken. (Reprinted with permission of Wiley-Blackwell from McCracken, T.O., Kainer, R.A., and Spurgeon, T.S. Spurgeon’s Color Atlas of Large Animal Anatomy. Baltimore: Lippincott Williams & Wilkins, 1999.)

In the antebrachium, the radius is consider­ably smaller than the ulna. These bones articu­late with the two bones of the carpus. The distal row of carpal bones have become fused with the proximal end of the metacarpals, and so these long bones of the metacarpal region are known as carpometacarpal bones, of which there are three. There are three digits. The first is often called the pollux and like the “thumb” of mammals possesses two phalanges.

There is considerable diversity in the appear­ance of the pelvis in birds, but like the mam­malian pelvis, three bones (ilium, ischium, and pubis) contribute to its construction. Like the thoracic limb, the pelvic limb shows a reduc­tion in the number of individual bones of the tarsus. Fusion of the proximal row of tarsal bones with the tibia creates the tibiotarsus. The fibula is fused with the tibiotarsus in its distal half. The distal tarsal bones are fused to a single weight-bearing metatarsal bone; this is there­fore correctly referred to as the tarsometatar- sus in birds. Poultry possess four digits; digit one projects in a medioplantar direction. The other three toes fan out in a forward direction. Digit one has two phalanges, digit two has three, digit three has four, and digit four has five.

The sternum of domestic poultry is dramati­cally modified for flight. As the pectoral muscles of birds are massively developed, the sternum (their primary attachment site) is correspond­ingly enlarged. The large, shieldlike mass of the sternum is further increased for muscle attach­ment by the presence of a large midline keel or sternal crest. The cranial sternum, especially, is adorned with projections that serve as articulat­ing processes and sites for muscle attachment.

Ten to 14 days prior to the beginning of a laying period, chickens develop a type of bony tissue unique to birds and crocodilians in the shaft of long bones such as the femur. This medullary bone consists of interlacing bony spicules, with blood sinuses and marrow between the spicules. Medullary bone may replace up to 75% of the hematopoietic tissue in laying hens, but it is absent in non-laying hens and males. Medullary bone formation is stimulated by the effects of estrogen on osteo­blasts, and just prior to the beginning of an egg production period, estrogen levels are relatively high as ovarian follicles mature and begin secreting estrogen. Medullary bone provides a ready source of calcium that can be mobilized later for use in eggshell production. Eggshell formation and calcium metabolism will be dis­cussed in more detail in a later section of this chapter.