THE FRONTAL BONES 



59 



parietal suture is usually closed at four years, the parieto-occipital at five years, and 

 the squamous at twelve to fifteen years. 



The Frontal Bones 



The frontal bones (Ossa frontalia) are situated on the limits of the cranium 

 and face, between the parietals behind and the nasal bones in front. Each is 

 HTegularly quadrilateral, and consists of naso-frontal, orbital, and temporal parts. 



The naso-frontal part (Pars naso-frontalis) forms the basis of the forehead. 

 Its external or frontal surface (Fades frontalis) is nearly flat, and is smooth and 

 subcutaneous; it iS separated from the temporal part 1jy the external frontal crest 

 (Crista frontalis externa). At the junction with the orbital part the supraorbital 

 or zygomatic process (Proc. zygomaticus) curves outward and downward to join 

 the zygomatic arch. The process partially separates the orbit from the temporal 

 fossa; its root is perforated by the supraorbital foramen (Foramen supraorbital), 

 or presents instead a notch on its anterior border; its upper surface is convex, while 

 the orbital surface is concave and smooth, forming a shallow fossa for the lacrimal 



9__ 



Fig. 37. — Left Frontal Bone of New-born Foal; Ventro-medial View. 

 A, Cerebral surface; fi, S, orbital part; C, temporal part; D, nasal surface; 1, surface of junction with opposite 

 bone; 2, frontal sinus; 3, ridge to which cribriform plate of ethmoid bone is attached; 5, fissure into which orbital wing 

 of sphenoid bone fits; 6, sphenoidal notch; 7, supraorbital process; 8, junction with nasal bone; 9, junction with pari- 

 etal bone; 10, junction with squamous temporal bone. 



gland (Fossa glandulse lacrimalis). The internal surface enters into the formation 

 of the cranial and nasal cavities. The two plates of the bone separate and diverge 

 in front, and thus inclose a large air-space which is part of the frontal sinus. The 

 internal plate curves downward and forward and joins the cribriform plate of the 

 ethmoid. Beyond this it inclines upward and joins the external plate at the naso- 

 frontal suture. The cerebral surface presents digital impressions for the cerebral 

 gyri. The nasal surface is longitudinally grooved. The external plate extends 

 forward and joins the nasal and lacrimal bones. 



The orbital part (Pars orbitalis) forms the major part of the medial wall of the 

 orbital cavity. It is separated from the naso-frontal part by a prominent ridge 

 which is part of the orbital margin. Its orbital surface (Facies orbitalis) is concave 

 and smooth, and presents superiorly a small depression (Fovea trochlearis) , which 

 is bridged by a small bar of cartilage,, around which the superior oblique muscle of 

 the eye is reflected. The lower border concurs with the orbital wing of the sphenoid 

 in the formation of the ethmoidal foramen. The nasal surface faces into the 

 frontal sinus and is to a small extent united with the lateral mass of the ethmoid. 



The temporal part (Pars temporalis) is separated from the orbital part by the 



