THE PTERYGOID BONES THE NASAL BONES 67 



The perpendicular part (Pars perpendicularis) is more extensive and forms 

 most of the lateral wall of the choanaj or posterior nares. The nasal surface (Facies 

 nasalis) is in the greater part of its extent concave and smooth, but presents a 

 narrow rough area to which the pterygoid bone is attached. Below this the bone 

 curves outward, forming the pterygoid process. The maxillary surface (Facies 

 maxillaris) presents three areas for consideration. The largest articulates with the 

 maxilla; it is rough and is crossed by a groove which concurs with one on the max- 

 illa in the formation of the palatine canal. Behind this is a smooth part which 

 assists in forming the pterygo-palatine fossa (Fossa pterygopalatina). The rough 

 area below this is overlapped by the pterygoid process of the sphenoid bone. The 

 dorsal border is perforated by the sphenopalatine foramen. Behind the foramen 

 the two plates of the bone separate to inclose part of the sphenopalatine sinus. 

 The inner plate curves medially to articulate with the vomer. The outer plate 

 joins the maxilla and frontal and the orbital wing of the sphenoid; it may join the 

 lacrimal bone also. 



Development. — The palatine bone ossifies in membrane from a single center. 



The Pterygoid Bones 



The pterygoid bones (Ossa pterygoidea) are narrow, thin, bent plates, situated 

 on either side of the posterior nares. Each has two surfaces and two extremities. 

 The medial surface is smooth, and forms part of the wall of the posterior nares. 

 The lateral surface articulates with the palatine, vomer, and sphenoid, concurring 

 with the last in the formation of the pterygoid canal. The ventral extremity is 

 free, turned slightly outward, and forms the hamulus pterygoideus ; this is grooved 

 externally and forms a pulley around which the tendon of the tensor palati muscle 

 is reflected. 



Development. — The pterygoid ossifies in membrane from a single center. 



The Nasal Bones _ 



The nasal bones (Ossa nasalia) are situated in front of the frontal bones and 

 form the greater part of the roof of the nasal cavity. They have an elongated 

 triangular outline, wide behind, pointed in front. Each presents two surfaces, two 

 borders, and two extremities. 



The facial surface is smooth and is convex transversely; the profile contour is 

 usually slightly wavy, with a depression about its middle and a variably prominent 

 area in front. 



The nasal surface is smooth and concave from side to side. About in its 

 middle it presents the dorsal turbinate crest (Crista turbinata dorsalis), which is 

 parallel with the medical border, and has the dorsal turbinate bone attached to it. 

 Most of this surface faces into the nasal cavity, but its posterior part, external to 

 the turbinate crest, enters into the formation of the frontal sinus; the latter area is 

 marked off by an oblique ridge which corresponds to the septum between the an- 

 terior and posterior parts of the dorsal turbinate bone. 



The medial border is straight, and meets the opposite bone at the nasal suture. 



The lateral border is irregular. Its anterior third is free and concurs with 

 the nasal process of the premaxilla in forming the naso-maxillary notch (Incisura 

 naso-maxillaris). Behind this it joins the end of the nasal process, the maxilla, 

 and the lacrimal, forming the naso-maxillary and naso-lacrimal sutures. 



The greater part of the edge is beveled and fits into a groove on the upper border of the 

 nasal process, the maxilla, and the lacrimal bone. 



- The posterior extremity or base is beveled and overlaps the frontal bone, 

 forming the naso-frontal suture. 



The anterior extremity or apex is pointed and thin. 



