THE SKULL THE CRANIUM 



213 



slightly convex ; it arches downward, forward, and inward. The rim 

 of the external auditory meatus is formed by the ectotympanic, with 

 the exception of a small area at the upper posterior part, which is 

 derived from the squamous. The rim is moderately sharp, and often 

 slightly serrated at the anterior, lower end. A notch at the upper 

 posterior part indicates the point at which, on the inner surface, the 

 septum of the bulla begins. 



The auditory meatus is irregularly egg-shaped, and almost twice 

 as long as it is high. It is rounded behind and pointed in front ; the 

 long axis is directed, from behind, obliquely downward and forward. 

 The external ear is attached near the rim of the meatus, and principally 

 to the ectotympanic. 



The interior of the bulla is divided into two chambers by a bony 

 septum (Figs. 145, 146) attached along a line indicated on the 



FIG. 145. 



Sagittal Crest. 



Wall of Cranium. 

 Cut Edge of Tentorium. 



Squamous. 



For Malleus. 



Aquxductus Fallopii. 



Fenestra Ovalis. 



For Incus. 



Cochlea. 



Inner Chamber of Bulla. 

 Upper Edge of Septum. 



Outer Chamber of Bulla. 

 Septum. 



Parietal. 



- Tentorium Cerebelli. 



Squamous of Temporal. 



Cells in Petrous. 



For TENSOR TYMPANI. 



H Fenestra Ovalis. 

 _Ja| Slit for STAPEDIUS. 

 ^^M Hri'li]*'. [Chambers of Bulla. 



Opening of Communication between Tito 

 ' Mnstoid of Temporal. 



Cochlea. 



Posterior Rim of External Auditory Meatus 



of Ectutymimnic of Bulla. 



Entoti/mpunic of j;/(//<i. 

 Groove in Basioccipital for Blood-vessel. 



Inner Chamber of Bulla. 

 TRANSVERSE VERTICAL SECTION OF SKULL THROUGH EXTERNAL AUDITORY MEATUS. 



outside by the line separating the ectotympanic and entotympanic 

 bones. The septum is directed upward and inward. At the upper 

 margin it is turned sharply outward, thus forming a flattened upper 

 surface, directed from in front backward, outward, and downward. 

 This surface is wide in front, where it rests on the promontory of the 

 petrous, and narrow and emarginate behind, where it forms the lower 



