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cavities connected to the respiratory pathways or the esophagus that are capable of filling with air but do not function in gas exchange in most vertebrate animals.
In tailless amphibians, air sacs are paired or unpaired processes in the back of the oral cavity called vocal sacs. In reptiles (some turtles and lizards) air sacs are blind processes in the lungs.
Birds have five pairs of air sacs. The abdominal sacs branch off from the main bronchi and are situated between the organs of the abdominal cavity. The other four pairs are extrapulmonary prolongations of secondary bronchi: (1) cervical, lying along the esophagus; (2) clavicular, frequently merging into a single interclavicular sac; (3) anterior thoracic, on the abdominal wall of the thorax; and (4) posterior thoracic, on the dorsal side. The main function of the air sacs in birds is to draw air through the lungs, especially during flight, to regulate heat, and to alter the specific gravity of the birds while swimming and diving. Many skeletal bones in birds (femur, humerus, sternum, and others) have cavities filled with air sac processes. Birds also have air sacs that are not connected to bronchi; the processes of these (pharyngonasal) air sacs in some birds extend to the cranial bones, under the skin, and into the anterior extremities.
Mammals have several kinds of air sacs: (1) some that originated as paired processes on the mucous membrane of the eustachian tubes (in horses, asses, zebras) and are situated on the neck in the region of the atlas; (2) paired and unpaired structures originating in the pharynx and serving to amplify sound (vocal sacs); (3) sacs that branch off from the widened posterior end of the trachea (in male striped seals) or from the esophagus (in male walruses) and serve to alter the specific gravity of the body; and (4) in the sperm whale, a blind air sac about 1 cu m in size that opens into the blow hole, where it collects air before a dive.
Plants have air sacs and cavities filled with air that originated as a result of the layers of the exine of pollen grains diverging. Air sacs are characteristic of the pollen of many gymnosperms, particularly conifers.
G. P. DEMENT’EV, A. N. DRUZHININ, and A. N. SLADKOV