200 MAMMALIAN ANATOMY 



ward, but at the basal angle it is truncated, giving rise to a short, 

 jagged portion, which faces directly backward and forms the anterior 

 boundary of the groove for the lateral sinus. The basal or superior 

 margin of the surface is not always clearly defined, but usually may 

 be distinguished as an elevated semicircular line separating the denser 

 petrous from the more porous mastoid. The most striking feature on 

 the posterior surface is the foramen for the transmission of the facial 

 and auditory nerves to the interior of the bone, known as the internal 

 auditory meatus. This foramen lies behind and below the centre of 

 the surface ; its general outline is oval, the long axis running nearly 

 parallel with the long axis of the skull, but forming an acute angle 

 with the long axis of the surface. On examining the foramen more 

 closely it is seen that, while its surrounding ridge is oval and more 

 elevated behind, its real outline is that of an inverted comma. The 

 large posterior round part receives the auditory nerve, and the small 

 anterior curved tail is the beginning of the canal for the facial nerve, 

 namely, the aquseductus Fallopii, which winds through the sub- 

 stance of the petrous, then between the petrous, mastoid, and squamous, 

 and opens on the outside and base of the skull at the stylomastoid 

 foramen. 



If the internal auditory meatus be examined more closely, its two 

 parts will be seen to be divided by a distinct longitudinal ridge 

 (Fig. 136). The lower part, for the auditory nerve, does not go all 



the way through the bone ; 



FIG. 136. ... ., -,. , f 



it is a deep pit. directed lor- 



Supenor Cribriform Spot.^^_ Entmn<x to Aqueductus 



Foramen Singulare -- IJX Fallopii. Ward and 111 Ward, and, DQ- 



coming somewhat smaller as 



Tract. 

 DIAGRAM OF LEFT INTERNAL AUDITORY MEATUS. " P r0cee dS HlWarCl, terilll- 



nates as a circular plate. 



The posterior wall of the pit and this circular end-plate form the 

 lamina cribrosa, which separates the meatus from the labyrinth within. 

 On the posterior wall is seen a small opening, the foramen singulare, 

 which leads to a canal, the posterior wall of which is perforated by 

 small holes, producing the inferior cribriform spot and transmitting 

 nerves to the lowest ampulla in the vestibule. In the lamina cribrosa, 

 in front of the foramen, is a depressed circular area, also sieve-like, 

 known as the middle cribriform spot, which transmits nerves to the 

 fovea hemisphrerica of the vestibule. From this area a perforated 



