Below the orbital cavity is the =pterygo-palatine fossa=. Its wall is formed by the pterygoid process, the perpendicular part of the palate bone, and the tuber maxillare. Its deep anterior part (maxillary hiatus) contains three foramina. The upper one, the =maxillary foramen=, is the entrance to the infraorbital canal, which transmits the infraorbital nerve and vessels. The =sphenopalatine foramen= transmits vessels and nerves of like name to the nasal cavity. The lower foramen, the =posterior palatine=, transmits the palatine artery and nerve to the palatine canal. The upper part of the fossa is smooth, and is crossed by the internal maxillary artery and the maxillary nerve. The lower part is chiefly roughened for the attachment of the internal pterygoid muscle, but is crossed in front by a smooth groove in which the palatine vein lies. The =maxillary= or =preorbital region= is formed chiefly by the maxilla, but also by the premaxilla, and the facial parts of the lacrimal and malar bones. Its contour is approximately triangular, the base being posterior. It offers two principal features. The =facial crest= extends forward from the lower margin of the orbit, and ends abruptly at a point about an inch and a half (3 to 4 cm.) above the third or fourth cheek tooth;[20] its inferior aspect is rough for the attachment of the masseter muscle. The =infraorbital foramen= is situated in a transverse plane about an inch (ca. 2 to 3 cm.) in front of the end of the crest and about two inches (5 cm.) above it. The foramen opens forward, and through it the infraorbital artery and nerve emerge. The surface over the premolar teeth varies greatly with age, in conformity with the size of the embedded parts of the teeth. In the young horse the surface here is strongly convex, the outer plate of bone is thin and even defective sometimes in places, and the form of the teeth is indicated by eminences (Juga alveolaria). In the old animal the surface is concave on account of the extrusion of the teeth from the bone. The downward curve of the premaxilla is pronounced in the young subject, very slight in the aged. The =inferior= or =basal surface= (Norma basalis), exclusive of the mandible, consists of cranial, guttural, and palatine regions.