THE MUSCULOSKELETAL SYSTEM

The avian skeleton is highly adapted for flight: it is light, compact, and strong and has a greater content of calcium phosphate than is found in mammalian bone. It is characterized by a prominent sternum, a pelvis that is open ventrally, a forelimb modified to form a wing, and considerable fusion of vertebrae (Figure 37-9, A-B).

A peculiar avian feature is the pneumatization of bones by air sacs, which are extensions of the lungs. The sacs are principally found in the body cavity, where they mingle with the viscera; however, they extend divertic­ula through pneumatic foramina into the medullary cavities of neighboring bones, which causes a consider­able part of the skeleton to be filled with air. Pneuma­tization is a gradual process achieved at the expense of the bone marrow. The process is most advanced in the best fliers, which thus obtain a skeleton that is large and strong without being correspondingly heavy. Much of the adult skull is also pneumatized, but the spaces there connect with airways in the head and not with the system of sacs. Another peculiarity is the appearance of (trabecular) medullary bone, the most important calcium reserve for egg production, before the laying season; the extra bone (polyostotic hyperostosis) may be mistaken for pathological processes on radiographs.