66 TEE CAT. [CHAP. iii. 



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

 foramen and bebind tbe posterior root of tbe zygoma. It is rougb 

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

 upper part of wbicb is formed hj tbe occipital bone. Opposite tbe 

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

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

 applied externally to tbe tympanic, and descending, immediately 

 bebind tbe stylo-mastoid foramen, to the pit for the tyrapano-hyal. 

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

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



On its inner aspect, below and bebind tbe squamous part (sq), 

 tbe temporal bone exhibits a triangular irregularly-shaped mass of 

 very dense osseous tissue. This is the ^jetroiis part of the temporal 

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

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

 canal leading to which — the meatus auditoriiis intcrnus — is the con- 

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

 This opening is di\ided within by a horizontal bony lamella into 

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

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

 the facial nerve, which thence proceeds to tbe st5do-mastoid foramen, 

 traversing in its way a canal termed tbe Aqueduct of Fallopius. 



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

 the inner wall of the petrosal, a depression or pit (e^), (which 

 lodges a process of tbe cerebellar part of tbe 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, tbe hiatus Fallopii, which transmits 

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

 Fallopius already mentioned. 



Just bebind tbe shallow depression above mentioned, and close to 

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

 opening, called the aquceductus rcstibuli. A still smaller aperture 

 j)laced close to and directly bebind the internal auditory opening (a-), 

 is the aqucductus cochlcce. Both these openings transmit small 

 veins of tbe internal ear. 



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

 sphcnoid is a largish opening {eu^, wbich is that of the EustacJiian 

 tube — a channel serAing to convey air from the mouth to tbe ear. 

 Tbe inner part of tbe canal is incompletely partitioned off by a small 

 bony lamella — (tbe processus coclileariformis). This rather con- 

 siderable Eustachian aperture is naturally roofed over and covered 

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

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

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

 a groove wbicb receives a venous canal, namely one of tbe two 

 branches into which the median venous channel (before noticed 

 as passing along beneath the median junction of the parictals) 

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



