66 THE CAT. [CHAP. in. 



and backwards (its apex being above) from above the stylo-mastoid 

 foramen and behind the posterior root of the zygoma. It is rough 

 externally, and forms the lower part of the lambdoidal ridge, the 

 upper part of which is formed by the occipital bone. Opposite the 

 posterior root of the zygoma it adjoins the par-occipital process, and 

 below that point of junction it narrows into a nipple-shaped process 

 applied externally to the tympanic, and descending, immediately 

 behind the stylo-mastoid foramen, to the pit for the tympano-hyal. 

 This triangular tract is the mastoidal region (m) of the temporal 

 bone, and the process just described is the mastoid process (ms). 



On its inner aspect, below and behind the squamous part (sq), 

 the temporal bone exhibits a triangular irregularly-shaped mass of 

 very dense osseous tissue. This is the petrous part of the temporal 

 bone, or the " petrosal." The petrous and mastoidal portions of the 

 bone enclose the inner and essential parts of the ear, the internal 

 canal leading to which the meatus auditorium internus is the con- 

 spicuous opening seen on the inner surface of the petrosal (mi). 

 This opening is divided within by a horizontal bony lamella into 

 two parts. The openings for the auditory nerve filaments are below 

 this horizontal lamella, while the opening above it gives entrance to 

 the facial nerve, which thence proceeds to the stylo-mastoid foramen, 

 traversing in its way a canal termed the Aqueduct of Fallopim. 



Above the opening of the internal auditory meatus there is, on 

 the inner wall of the petrosal, a depression or pit (cb\ (which 

 lodges a process of the cerebellar part of the brain) surmounted by 

 a prominence which indicates the place of the anterior vertical 

 semi-circular canal of the internal ear. Below and in front of this 

 prominence is a small foramen, the hiatus FaUopii, which transmits 

 the superficial petrosal nerve, and leads back into the Aqueduct of 

 Eallopms already mentioned. 



Just behind the shallow depression above mentioned, and close to 

 the posterior margin of the petrosal, is a small vertically elongated 

 opening, called the aquceductus vcstibuli. A still smaller aperture 

 placed close to and directly behind the internal auditory opening (a;), 

 is the aqueductus cochlece. Both these openings transmit small 

 veins of the internal ear. 



Between the anterior part of the petrosal bulla and the ali- 

 sphenoid is a largish opening (eu) f which is that of the Eustacliicui 

 tube a channel serving to convey air from the mouth to the ear. 

 The inner part of the canal is incompletely partitioned off by a small 

 bony lamella (the processus cochleariformis). This rather con- 

 siderable Eustachian aperture is naturally roofed over and covered 

 by a backward expansion of the alisphenoid. On the inner side 

 of it, at the hinder portion of the junction of the squamosal and 

 petrosal portions of the temporal bone, on its inner surface, is 

 a groove which receives a venous canal, namely one of the two 

 branches into which the median venous channel (before noticed 

 as passing along beneath the median junction of the parietals) 

 divides as it descends. This groove may lead into a canal opening 



