THE TEMPOEAL BONES. 125 



At birth the occipital consists of four parts the interparietal and supra-occipital 

 combined, the basi-occipital, and the exoccipitals one on either side. 



Ossa Temporalia. 



The temporal bone lies about the centre of the inferior half of either side 

 of the skull, and enters largely into the formation of the cranial base. It is 

 placed between the occipital behind, the parietal above, the sphenoid in front, and 

 the occipital and sphenoid medially and below. At birth it consists of three parts 

 a superior and lateral part, the squama temporalis or squamous portion ; a medial 

 and posterior portion, the petro-mastoid, which contains the parts specially associated 

 with the sense of hearing, together with the organ associated with equilibration ; 

 and an inferior or tympanic part, from which the floor and anterior wall of the 

 external acoustic meatus is formed. 



The squamous part consists of a thin shell-like plate of bone placed 

 vertically, having a medial (cerebral) and a lateral (temporal) surface and a 

 semicircular upper border. Inferiorly, behind, and medially it is fused in early 

 life with the petro-mastoid portion by means of the squamoso-mastoid and the 

 petro-squamosal sutures, traces of which are often met with in the adult bone ; 

 whilst below and in front it is separated from the tympanic and petrous parts by 

 the petro-tympanic fissure. Its temporal surface, smooth and slightly convex, enters 

 into the formation of the floor of the temporal fossa, and affords attachment to 

 the temporal muscle. Near its posterior part it is crossed by one or more ascending 

 grooves for the branches of the middle temporal artery. In front and below there 

 springs from it the processus zygomaticus. This arises by a broad attachment, 

 the surfaces of which are inferior and superior ; curving laterally and forwards, it 

 then becomes twisted and narrow, so that its sides are turned medially and 

 laterally and its edges directed upwards and downwards. Anteriorly it ends in an 

 oblique serrated extremity which articulates with the temporal process of the 

 zygomatic bone. Posteriorly the edges of the zygomatic process separate and are 

 termed its roots. The superior edge, which becomes the posterior root, sweeps back 

 above the external acoustic meatus, and is continuous with the supra-mastoid 

 crest, which curves backwards and slightly upwards, and serves to define the limit 

 of the temporal fossa posteriorly. Internally this ridge corresponds to the level of 

 the floor of the middle cerebral fossa. The inferior edge turns medially and 

 constitutes the anterior root; the inferior surface of this forms a transversely 

 disposed rounded ridge, the tuberculum articulare (O.T. articular eminence), behind 

 which there is a deep hollow, the fossa mandibularis, limited posteriorly by the 

 tympanic plate, and crossed at its deepest part by an oblique fissure, the petro- 

 tympanic fissure. This cleft, which is closed laterally, transmits about its middle 

 the tympanic branches of the internal maxillary artery, and lodges the anterior 

 process of the malleus. At its medial end the lips of this fissure are frequently 

 separated by a thin scale of bone, a downgrowth from the tegmen tympani 

 of the petrous part, which here separates the tympanic from the squamous elements, 

 forming in its descent the major part of the lateral wall of the osseous auditory 

 tube, which lies just medial to it. Between this scale of bone and the posterior 

 edge of the fissure there is a canaliculus, which transmits the chorda tympani 

 nerve. The part of the mandibular fossa in front of the petro-tympanic fissure, 

 as well as the articular tubercle, articulates with the condyle of the mandible, 

 through the medium of the interposed articular disc. The part of the fossa 

 behind the fissure is non-articular and lodges a portion of the parotid gland. At 

 the angle formed by the divergence of the two roots of the zygoma, in correspond- 

 ence with the lateral part of the articular tubercle, there is a rounded tubercle ; 

 to this are attached the fibres of the temporo-mandibular Ligament of the mandibular 

 joint. In front of the medial end of the articular tubercle there is a small 

 triangular surface, limited anteriorly by the edge of the anterior root, and medially 

 by a thick serrated margin which articulates with the temporal aspect of the 

 great wing of the sphenoid ; this area forms part of the roof of the infra-temporal 

 (O.T. zygomatic) fossa. Just anterior to the external acoustic meatus and projecting 



