54 THE SKELETON OF THE HORSE 



are irregularly quadrilateral in outline. The temporal surface (Facies temporalis) 

 enters into the formation of the infratemporal fossa, and bears the pterygoid process 

 on its anterior part; at the junction with the body there is a small groove which 

 leads forward to the pterygoid canal. The cerebral surface (Facies cerebralis) 

 presents, at the junction with the body, two longitudinal grooves (Sulci nervorum). 

 The lateral groove is the larger, and leads forward to the foramen rotundum; it 

 contains the maxillary nerve. The medial groove conducts to the foramen orbitale, 

 and contains the cavernous sinus of the dura mater. The outer groove is bounded 

 laterally by a thin overhanging crest, on which is a small groove for the fourth nerve. 

 The remainder of the surface is concave and supports the pyriform lobe of the brain. 

 The dorsal border joins the squamous temporal at the spheno-squamous suture. 

 The anterior border joins the orbital wing above; below this it is free and forms the 

 pterygoid crest (Crista pterygoidea) . The crest is continued on the pterygoid 

 process; on or under its upper part there is usually a small opening, the trochlear 

 foramen (Foramen trochleare) . Just behind the crest is the foramen alare parvum,"- 

 through which the anterior deep temporal artery emerges from the alar canal of 

 the pterygoid process. The posterior border forms the anterior boundary of the 

 foramen lacerum; it presents three notches, which are, from within outward, the 

 carotid, oval, and spinous (Incisura carotica, ovalis, spinosa). The angle of junc- 

 tion of the dorsal and posterior borders articulates with the parietal bone. 



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)," which trans- 

 mits the internal maxillary artery. From this canal a branch leads upward and 

 forward to open at the foramen alare parvum. The lateral surface is concave, and 

 is marked by lines for muscular attachment. The medial surface is convex; it is 

 largely concealed by the overlapping palate and pterygoid bones. 



The pterygoid canal (Canalis pterygoideus)' continues the groove noted on the 

 ventral surface of the temporal wing at its junction with the body. It extends for- 

 ward and upward between the root of the pterygoid process, the presphenoid, and 

 the pterygoid bone, and opens in the posterior part of the pterygo-palatine fossa. 

 It transmits the nerve of the pterygoid 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. 



In the new-born foal the unossified dorsal part of the orbital wing fits into a hiatus of the 

 frontal bone; in some cases it comes 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 bone (Os ethmoidale) hes in front of the body and orbital wings 

 of the sphenoid. It projects forward between the orbital parts of the frontal bones 

 and enters into the formation of the cranial, nasal, and paranasal cavities.^ It 

 consists of four parts — the cribriform plate, two lateral masses, and the perpen- 

 dicular plate. 



The cribriform plate (Lamina cribrosa) is a sieve-like partition between the 



• Also termed the temporal foramen. 



^ This is also called the subsphenoidal canal or pterygoid foramen. 



' Also known as the Vidian canal . 



■• On account of its deep situation and the fact that it cannot be separated from its surround- 

 ings, the ethmoid must be studied by means of appropriate sagittal and transverse sections of the 

 skull. 



