On the median pharyngeal valve of the American alligator (Alligator mississippiensis).

The middle ear cavities of crocodilians have complex connections with the pharyngeal lumen, including lateral and median components which both open into a single chamber located on the dorsal midline of the pharynx. This chamber and the surrounding soft-tissue is herein termed the median pharyngeal valve. In the American alligator (Alligator mississippiensis) this valve opens, for a duration of 0.3 s, approximately every 120 s; the patency of the median pharyngeal valve was not influenced by either auditory stimuli or by submersing the alligator underwater. The median pharyngeal valve has an outer capsule of dense connective tissue and fibrocartilage and an inner "plug" of loose connective tissue. These opposing surfaces are lined by respiratory epithelium and separated by a cavity that is continuous with the middle ear cavities and the pharyngeal lumen (through a central opening in the capsule termed the pore). The inner plug of the median pharyngeal valve is contacted by skeletal muscles positioned to serve as both elevators/retractors (which would open the valve) and elevators/protractors (which, in conjunction with gravity, would close the valve). Unlike other vertebrate valve systems, the median pharyngeal valve appears to function as a deformable ball check valve.