the frontal bones — the temporal bones 55 



The Frontal Bones 



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

 and face, betwcM'u the parietals l;)ehind and the nasal bones in front. Each is 

 irregularly (luadrilateral, and consists of frontal, orl)ital, and temporal parts. 



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

 external surface (Facies frontalis) is nearly flat, and is smooth and subcutaneous; 

 it is separated from the temporal part ])y 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, or presents instead a notch on its anterior 

 border; its upper surface is convex, while the lower or orbital surface is concave 

 and smooth, forming a shallow fossa for the lacrimal gland (Fossa glandulae lacri- 

 malis). The internal surface enters into the formation of the cranial cavity and 

 the frontal sinus. The cranial surface presents digital impressions for the cerebral 

 gyri. The two plates of the bone separate and diverge in front, and thus inclose 

 a large air-space which is part of tlie frontal sinus. The cranial plate curves down- 

 ward and articulates with the cribriform })late of the ethmoid bone; the facial plate 

 extends forward and joins the nasal and lacrimal bones. 



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

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

 part of the orl)ital margin. Its external or orbital surface 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 or internal orbital foramen. The internal surface faces 

 into the frontal sinus and gives attachment to the lateral mass of the ethmoid. 



The temporal part is separated from the orbital part by the deep sphenoidal 

 notch (Incisura sphenoidalis), which is occupied by the orbital wing of the sphenoid. 

 Its external surface forms part of the inner wall of the temporal fossa. The 

 internal surface is largely covered l;)y the orbital wing of the sjihenoid in the young 

 subject, but later forms part of the wall of the frontal sinus. 



The principal connections of the frontal bone are as follows: (1) The inner border joins its 

 fellow at the frontal suture. (2) The anterior Ijorder meets the nasal and lacrimal at the naso- 

 frontal and fronto-lacrimal sutures. (3) Laterally it forms the spheno-frontal suture with the 

 orbital wing of the sphenoid, and also joins the palate bone and maxilhi. (4) Posteriorly it 

 meets the parietal at the parieto-frontal (or coronal) suture, and articulates below this with the 

 squamous temporal. (5) The extremity of the supraorbital process unites with the zygomatic 

 process of the temporal bone. 



Development. — Each ossifies in membrane from one center which appears in 

 the root of the supraorbital process. In the new-born foal there is a slit between 

 the cranial plate and the orbital and temporal plates which receives the cartilagin- 

 ous margin of the orbital wing of the sphenoid. 



The Temporal Bones 



The temporal bone (Os temporale) forms the greater part of the lateral wall 

 of the craniuuL It is situated between the occipital behind, the parietal above, 

 the frontal in front, and the sphenoid below. It consists of two distinct parts, 

 squamous and petrous. 



1. The squamous temporal (Squama temporalis) is a shell-like plate which has 

 two surfaces and four l)orders. 



The internal surface (Facies cerebralis) is largely overlapped by the surround- 

 ing bones, l)ut its central part is free and presents digital impressions and vascular 

 grooves. 



