Maxillar recess

In Glires and Carnivorous, the maxillary sinus is not much developed; it forms a simple maxillary recess (Recessus maxillaris), broadly opened in the nasal cavity. Still not very developed in Pigs, it is much vaster and complicated in Equidae and Ruminants, especially in Bulls. In Ungulates, the infra-orbital canal subdivides it into two compartments, lateral and medial, that communicate dorsally to it.

In Equidae, the conchofrontal sinus and the sphenoidal sinus end in its medial part, that communicates alone with the bottom of the middle meatus by an elongated slit. Besides, the compartment is in great part formed by a bullous extension of the ventral concha. the horses present an exceptional particularity that is not found in the donkeys: a transversal sheet that subdivides the maxillary sinus into two parts, described respectively as a caudal and a rostral maxillary sinus.

In Bulls, this sinus is still vaster and more complicated: Its medial part extends a long way inside the palatine process of the maxilla and in the palatine bone, forming what is described as a palatine sinus (Sinus palatinus); another extension goes until the medial margin of the orbit and forms the lacrimal sinus (Sinus lacrimalis). In this species, there is no communication with the frontal or the sphenoidal sinus.