OSTEOLOGY. 1 5 



ing the orbital arch and articulating externally with the zygomatic 

 process of the temporal. An opening in the orbital arch, the 

 supra-orbital. 



At the base of the arch a depression for the pulley of the 

 superior oblique. 



The Internal Surface is concave and shows — 



A superior part articulating with the parietal above and joining 

 below the crista galH and wing of the sphenoid. 



An inferior part articulating in the median line with the 

 ethmoid, and shows two large openings into \\\q. frontal sinuses, 

 two spaces between the external and internal plates of the bone. 

 It forms part of the roof of the nasal fossae. 



The supei'ior border articulates with the parietal and temporal, 

 the inferior with the lachrymal and nasal, and the lateral with 

 the palate and sphenoid, forming with the latter the orbital 

 foramen. 



The frontal has two centres of ossification. 



Articulations, 14. 



With two parietal, two temporal, two nasal, two lachrymal^ 

 sphenoid, ethmoid, two palate and two superior maxillary. 



ETHMOID. 



The ethmoid (sieve-like) lies at the anterior part of the floor 

 of the skull, and forms part of the roof of the nasal fossae. 



It is divided into a vertical plate, a horizontal portion, and two 

 lateral masses. 



The vertical portion articulates below with the vomer and the 

 median nasal cartilage, forms the median septum of the nose, and 

 terminates above in the crista galli, a small elevation on which a 

 process of the cerebral dura mater is attached. 



The horizontal portion forms the roof of the nasal cavities, part 

 of the cranial floor, and is perforated by a number of openings 

 for the passage of the filaments of the olfactory nerves. The 

 lateral masses form part of the orbital and nasal walls, are exca- 

 vated to form the ethmoidal cells and show an anterior projection 

 forming the superior turbinated bone which is really a part of the 

 ethmoid and considered as such. 



Articulations, 7. 



Sphenoid, frontal vomer, two palate, two superior maxillary. 



