Sedlmayr, J.C.*; Witmer, L.M.: The Avian Suborbital Air Sac and its Potential Role in Brain Cooling in Birds and Other Dinosaurs

The avian suborbital air sac, a caudal diverticulum of the antorbital paranasal air sinus, partially encircles the eyeball and interleaves between portions of the jaw musculature. It is unique in being a paranasal sinus capable of being actively ventilated like a bellows pump during abduction and adduction of the mandible. The discovery of a dense venous plexus surrounding the suborbital air sinus suggests a physiological role as an important contributor to selective brain cooling via its drainage into the ophthalmic rete, an arteriovenous countercurrent heat exchanger known to cool the hypothalamus. In addition, the suborbital plexus is potentially involved in whole-brain cooling via multiple routes of drainage, including: (1) the ethmoid vein and thus the dorsal sagittal sinus; (2) drainage through foramina for CN II-VI into the cavernous sinus which surrounds the hypophysis; and (3) along the maxillomandibular (trigeminal) nerve via drainage of the ophthalmic rete, into the rostral petrosal sinus which is in close contact with the mesencephalon. These findings may shed light on dinosaur physiology in that several non-avian theropod dinosaurs possess osteological correlates for the suborbital air sinus; moreover, many fossil archosaurs display the bony architecture permitting the presence of a suborbital diverticulum of the antorbital air sinus. The very large size of the cranial nerve foramina in certain dinosaurs may provide further evidence of the brain-cooling properties of the suborbital air sac and/or other orbital vascular structures.