234 MAMMALIAN ANATOMY 



just posterior to the tensor tympani fossa and dividing the fenestrse 

 (Fig. 167), the anterior wall of the vestibule maybe seen more clearly, 

 and also the beginning of the lamina spiralis at the lower outer part ; 

 its line of attachment to the cribriform plate of the internal auditory 

 rueatus ; and the manner in which it separates the vestibule from the 

 scala tympani. 



If the anterior aspect of this section (Fig. 168) be examined, the 

 posterior part of the vestibule can be studied. Owing to the oblique 



FIG. 108. 



Ridge for Superior Semi- _ 

 circular Canal. 



Superior and Posterior 

 Semicircular Canals. 



. . - Fallopii. 



Groove to External Semi-f/ .\ ^jmi^TT. External Ampulla 



circular Canal. ^ / '~" -=" s ^ *'' 



Posterior Ampulla. - 



Posterior Rim of Fenestra Rotunda. 



SECTION OF LEFT PETROUS TRANSVERSE TO LONG AXIS. ANTERIOR 

 ASPECT. (Enlarged Three Times.) 



position of the petrous in the skull, the outer as well as the posterior 

 wall is exposed. The upper inner part of the outline is emarginate 

 because it is a section of the floor of the appendicular fossa, behind 

 which is the swelling for the superior semicircular canal ; the upper 

 outer part is a section of the anterior surface. The lateral outline 

 shows above a section of the tensor tympani fossa and the slit-like 

 section of the aqureductus Fallopii. Below these features the outline 

 is deeply incised by the fossa for the fenestra ovalis, and at the bottom 

 of the section is the fenestra rotunda. 



At the top of the vestibule is the divided superior ampulla ; below 

 it, the external ampulla. At the medial lower corner is the divided 

 posterior ampulla, and, leading clown into it, the groove for the non- 

 ampullar end of the external canal. The long channel extending 

 upward and inward from the upper outline of the vestibular wall is 

 a section of the common canal for the non-ampullar ends of the 

 superior and posterior semicircular canals. All these openings are 

 denned by ridges ; the actual posterior wall of the vestibule is confined 

 to a small central area (Fig. 163). The feuestra rotunda appears to 

 open into the vestibule, because the lamina spiralis has been destroyed. 



