THE SKULL THE CEANIUM 



231 



outward, backward, and upward into the external semicircular canal. 

 This canal can be easily opened and its course traced behind the 

 stapedius fossa to its lower end, namely, the groove on the convex 

 lateral dorsal wall of the posterior ampulla. 



Crossing the anterior wall of the vestibule from the upper rim of 

 the fenestra ovalis, in front of the superior ampulla, is a porous pyram- 

 idal elevation, known as the vestibular crest. Above, anterior and 

 lateral to the crest, is a depression, the fovea hemi-elliptica, which with 

 the crest itself is pierced by the foramina from the superior cribriform 

 spot of the auditory meatus ; behind, below, and medial to the crest 

 is a larger fovea hemisphserica, pierced by foramina from the middle 

 cribriform spot. In the concave lower medial wall of the posterior 

 ampulla is a group of fine openings from the posterior wall of the 

 foramen singulare. All these openings transmit branches of the audi- 

 tory nerve to the vestibule. In the metal cast they are indicated by 

 patches of granulations (Fig. 160). 



The relation of these features to the auditory meatus can be best 

 seen by carefully grinding away a portion of the posterior surface 

 (Fig. 164) of the petrous. The superior and posterior semicircular 



FIG. 164. 



<^v 



Posterior Semicircular Canal, i 



X. 



Superior Semicircular Canal. 

 Petrous Cells. 

 Aquxductus Fallopii. 

 Hiatus Fallopii. 



--Scala Vestibuli. 



;<$ ^ 



IA/*5 

 <?- ? 



v? 



'-A j 



Superior Cribriform Spot .%. - - y,- 



x r '- 

 Superior and Posterior Canal. * 



Foramen Singulare. - -^ - -^- -~^- 



Middle Cribriform Spot 



Aquxductus Cochlex, 



Spiral Cribriform Tract. '" 

 Scala Tympani.-' 



LEFT PETROUS. MEDIAL ASPECT. (A PORTION OF POSTERIOR SURFACE REMOVED.) 



(Enlarged Four Times.) 



canals will be first opened ; then the aquoeductus Fallopii and its 

 anterior branch to the hiatus will be exposed ; finally the terminal 

 whorls of the cochlea become visible, and the aqureductus cochleae is 

 cut across. 



If the grinding be carried still further (Fig. 165), the inner and 



