46 OSTEOLOGY, OR ANATOMY OF OSSEOUS SYSTEM 



sellse, and completes a foramen for the inferior petrosal sinus 

 and sixth nerve. The anterior border has two parts an outer, 

 forming the petrosquamous fissure, and an inner free portion 

 to form the petrosphenoidal suture. The inferior border is 

 largely concealed by the tympanic and petrous portions; near 

 the apex it is indistinct, and here the bone is rather three- 

 sided. 



The tympanic portion is beneath the petrous and between 

 the mastoid and squamous. At birth it is a ring from which 

 is developed the tympanic plate. The thickened outer extremity 

 of this plate is the external auditory process, a curved, uneven 

 lamina forming the anterior and inferior wall of the external 

 auditory meatus and tympanum. The upper margin of the 

 plate is concealed by the petrous and forms the posterior 

 boundary of the fissure of Glaser. Its lower margin descends 

 as a sharp edge, the vaginal process; it is continuous with the 

 inferior border of the petrous portion. 



The Sphenoid Bone 



The sphenoid bone (wedge-like) is placed across the base 

 of the skull near its middle, and binds the other cranial bones 

 together. It helps form the cavities of the cranium, orbits, 

 and nasal fossse, and has to do with six pairs of cranial nerves. 

 It resembles a bat with outstretched wings, and consists of 

 a body, greater and lesser wings, and pterygoid processes. It 

 articulates with twelve bones, all those of the cranium, and 

 five of the face; posteriorly with the occipital and temporals, 

 anteriorly with the ethmoid, palatals, frontal, and malars; 

 laterally with the temporals, frontal, and parietals; inferiorly 

 with the vomer and palatals, and sometimes with the superior 

 maxillse. 



The body is followed into two cavities separated by the 

 sphenoidal septum, and opening anteriorly into the upper and 

 back part of the nasal fossae behind the superior turbinate 

 bone. 



The superior surface presents in front the ethmoidal spine, 

 articulating with the cribriform plate of the ethmoid. On 

 either side of this surface is a slight depression for the olfactory 

 lobe, and its posterior margin is a transverse ridge the limbus 

 sphenoidalis. Behind this, on a lower plane, is the optic groove, 



