THE SKULL THE CRANIUM 229 



(Fig. 161). The posterior, larger cavity is the beginning of the 

 scala tympani ; its roof is formed of the modiolus and the lamina 

 spiralis. In the medial posterior corner is the opening of the aqua> 

 ductus cochleae ; at the lateral posterior part is the beginning of the 

 slit between the lamina spiralis and the wall. At the anterior medial 

 part the scala curves dorsally and laterally and disappears to complete 

 the first turn. In front of the lamina spiralis, which in this part of 

 the cochlea is wide and strongly curved, is the beginning of the scala 

 vestibuli. It is narrower and not so deep ; it lies in a lower plane 

 than the scala tympani, and is directed almost transversely to the long- 

 axis of the skull. Its outer end turns upward and medially into the 

 vestibule, just opposite the fenestra ovalis ; its medial end turns upward 

 and laterally and forward around the modiolus. 



The anterior whorls of the cochlea should be opened by break- 

 ing away the ventral wall of the smaller swelling of the promon- 

 tory. It will be observed that the scala tympani which began as 

 the wider of the two scalar has now become the narrower. On the 

 cochlear wall opposite the free edge of the lamina spiralis is the lamina 

 spiralis secunda. A second spiral line is sometimes present ; it marks 

 the attachment of another membrane. The spiral lamina ends in a 

 hook-like edge in the anterior whorl. 



The vestibule can be examined in the same specimen by removing 

 its floor, the lamina spiralis, and the lateral portion of the modiolus 

 in the first cochlear whorl (Fig. 162). The spiral area produced by 

 breaking away the attached edge of the lamina is pierced by fine 

 openings, which are the foramina of the posterior part of the spiral 

 cribriform tract of the internal auditory meatus. Unless great care 

 be observed, this rough area will be destroyed and a large opening 

 made into the meatus. 



The walls of the vestibule enclose an irregular ovoid cavity which 

 is apparently bent forward so that the anterior wall is concave and the 

 posterior wall convex from above downward. (Compare Fig. 163.) It 

 is so inclined in the petrous that its upper part is nearer the side of the 

 skull than the lower part, and its posterior part is nearer the middle 

 line than the anterior part ; its inner wall, therefore, in part overhangs 

 the cavity, and the lower part of its outer wall is below the cavity ; the 

 upper part of the outer wall is pierced by the fenestra ovalis. The 

 posterior wall is formed by the crescentic bony mass anterior and 



