188 BONES OP THE CKANIUM. 



for the convolutions of the brain. Its superior ridge con- 

 tains a groove for lodging the superior petrosal sinus. 



The posterior surface has about its centre a large open- 

 ing, the meatus auditorius internus, which gives passage to 

 the seventh pair of nerves. It is directed outwards and 

 somewhat forwards into a short canal, at whose termina- 

 tion there is a transverse ridge dividing it into two parts. 

 The inferior is cribriform and transmits the portio mollis 

 or auditory nerve, while the superior is a single foramen, 

 which leads to the aqueduct ofFallopius, and gives passage 

 to the portio dura or the facial nerve. This aqueduct of 

 Fallopius is a long canal passing outwards and downwards 

 behind the tympanum and terminating in the stylo-mastoid 

 foramen. Behind the meatus internus is a small orifice, 

 the aqueduct of the vestibule. The base or exterior mar- 

 gin of the petrous portion is rough, for the attachment of 

 the cartilage of the ear, and at this point is seen the fora- 

 men auditorium externum which leads into the auditory ca- 

 nal, a tube about a half an inch long that takes a curved 

 direction downwards,, inwards, and forwards, to the mem- 

 bran a-tympani. This canal is composed chiefly of what is 

 called the auditory process. The petrous bone also contains 

 the organs of hearing, which will be examined in another 

 place. 



The foramina are twelve in number, 10 special and 2 com- 

 mon. The special are the external and internal auditory, 

 the stylo-mastoid, the mastoid, the carotid, glenoidal, Eusta- 

 chian, Vidian, aqueductus cochleae, and vestibuli. The com- 

 mon are the anterior and posterior foramina lacera. 

 The processes enumerated are six the styloid, the mastoid, 

 the auditory, the vaginal, the zygomatic, and the jugular. 

 The structure of the temporal bone, in its squamous por- 

 tion, is thin and mostly compact; the mastoid contains 

 large cells, and the petrous is considered next in density 

 to the teeth. Its development takes place by six points, viz : 

 the squamous, mastoid, petrous, zygomatic, styloid, and 

 auditory. 



The first osseous point is seen in the squamous about the 



