The =pterygoid processes= (Processus pterygoidei) arise from the temporal wings and the body. They project downward and forward, and curve outward at the lower part. The root is perforated by the =alar canal= (Canalis alaris),[11] which transmits the internal maxillary artery. From this canal a branch leads upward and forward to open at the temporal foramen. The =external surface= is concave, and is marked by lines for muscular attachment. The =internal surface= is largely concealed by the overlapping palate and pterygoid bones, with which it concurs in the formation of the =pterygoid= or =Vidian canal=. =Development.=—The sphenoid is ossified in cartilage, and consists in early life of two distinct parts, the presphenoid and postsphenoid. The former develops from two centers, one in each wing; the latter has three centers, one for the body and one for each wing. The pterygoid processes ossify from the centers of the temporal wings. =Variation.=—The dorsal border of the orbital wing may come to the surface through a defect in the frontal bone at the place where the horn process is situated in animals which have frontal horns. THE ETHMOID BONE The =ethmoid= (Os ethmoidale) is situated in front of the body and orbital wings of the sphenoid. It projects forward between the orbital plates of the frontal bones and enters into the formation of the cranial, nasal, and paranasal cavities.[12] It consists of four parts—the cribriform plate, two lateral masses, and the perpendicular plate. The =cribriform plate= (Lamina cribrosa) is a sieve-like partition between the cranial and nasal cavities. Its margin joins the orbital wings of the sphenoid laterally, and the cranial plate of the frontal bones dorsally. Its =cranial surface= is divided into two parts by a median ridge, the =crista galli=, which is the intracranial portion of the perpendicular plate. Each half forms a deep oval cavity, the =ethmoidal= or =olfactory fossa=, which lodges the olfactory bulb. The plate is perforated by numerous small foramina for the passage of the olfactory nerve filaments, and on either side is the much larger =ethmoidal foramen.= The =nasal surface= is convex, and has the lateral masses attached to it.