Now that the organs and air sacs have been described, a brief account of how the body cavity (celom) is subdivided may be helpful.

In birds no diaphragm separates thoracic from abdominal organs. However, the body cavity is divided into three parts by horizontal and oblique septa, which are mostly thin and translucent but may contain some fibrous tissue.

The horizontal septum is attached laterally to the ribs and medially to the bodies of the thoracic vertebrae; caudally it makes contact with the oblique septum. It forms the ventral surface of paired cavities that are bounded laterally and dorsally by the ribs and thoracic vertebrae. These spaces contain the lungs.

The larger oblique septum is attached to the sternum ventrally, the sixth and seventh ribs laterally, and the horizontal septum and thoracic vertebrae dorsally. It forms the caudoventral surface of paired cavities that are bounded dorsally by the horizontal septum and laterally by the thoracic and abdominal wall. This part of the body cavity contains the thoracic air sacs and the thoracic parts of the cervical and clavicular air sacs.

The largest of the three parts is caudal to the oblique septum. It is bounded dorsally by the pelvis, dorsocranially by the oblique septum, and ventrally by the caudal portion of the sternum and abdominal muscles. It contains the heart, liver, spleen, gastrointestinal and urogenital tracts, and abdominal air sacs. It is further divided by mesenteries and peritoneal folds, resulting in a complex set of compartments.