228 



MAMMALIAN ANATOMY 



in the macerated bone are in communication alone 1 the free edsre of 



O O 



the lamina spiralis, but in the recent state are shut off from each 

 other by a membrane stretching from this free edge to a spiral ridge, 

 the lamina spiralis secunda, on the Avail of the cochlea. This ridge 

 appears on the cast as a delicate spiral groove. 



One of the scalse opens behind and above into the vestibule ; hence 

 it is called the scala vestibuli. The other scala ends in an enlargement 

 under the vestibule, which in the macerated skull opens freely into 

 the tympanum through the fenestra rotunda ; it is therefore called the 

 scala tympani. It communicates also with the cranial cavity through 

 the narrow aqureductus cochleae. 



FIG. 161. 



Fossa for TENSOR TYMPANI _ 

 Bridge 



Fenestra Ovalis. 

 Opening into Vestibule 



Third morl of Cochlea. 



Second WJiorl of Cochlea 



Aquxductus Fallopii. 

 --Fossa of STAPEDIUS. 



" ' - Fenestra Rotunda. 



""*- Scala Tympani. 

 Scala Vcstibuli. 



LEFT PETROUS. INFERIOR ASPECT. (A PORTION OF INFERIOR SURFACE REMOVED.) 



(Enlarged Four Times.) 



The first spiral turn or whorl of the cochlea is indicated on the 

 inferior surface of the petrous by the promontory proper ; the other 

 two whorls, which are much smaller, are contained in the swelling 

 medial to the fossa for the tensor tympani muscle. 



If the petrous be removed from the rest of the temporal bone, the 

 student can see, by looking into the fenestra rotunda, the posterior 

 part of the scala tympani in the first whorl, and just within the lateral 

 part of the rim of the fenestra, between the edge of the lamina spiralis 

 and the wall of the cavity, the slit which leads into the scala vestibuli 

 and its posterior prolongation, the vestibule itself. 



If the inferior wall of the promontory be carefully broken away, 

 the ventral half of the first whorl of the cochlea will be then exposed 



