42 THE BONES OF THE HEAD. 



tympanum, while the other looks towards the glenoid fossa. The upper margin of 

 the tympanic plate sinks beneath the squamous, and forms the posterior boundary 

 of the fissure of Glaser ; while its lower margin descends as a sharp edge, the 

 vaginal process, which partly surrounds the sfcyloid process at its base. The styloid 

 process is long and tapering, and is directed downwards and forwards. It is placed 

 in front of the digastric fossa, and has immediately behind it the foramen which 

 forms the outlet of the canal of the facial nerve, named stylo-mastoid from its 

 position between the styloid and mastoid processes. Internal to the stylo-mastoid 

 foramen is a small irregular surface, the jugular facet, which articulates by syn- 

 chondrosis with the jugular process of the occipital bone. In front of this comes 

 a smooth and deep depression, the jugular fossa, which forms with the jugulai 

 notch of the occipital bone the jugular foramen. In front of the jugular fossa is 

 the carotid foramen, the inferior extremity of the carotid canal ; and internal to the 

 carotid foramen is a rough, free surface which is continued into the inner 

 extremity, or apex of the petrous bone. The carotid canal ascends at first perpen- 

 dicularly, then turns horizontally forwards and inwards, and emerges at the apex, 

 close to the anterior margin ; it transmits the internal carotid artery. 



The posterior surface looks backwards and inwards, and forms part of the 

 posterior fossa of the base of the skull. About the centre of this surface is a large 

 orifice leading into a short canal which is directed outwards, the internal auditory 

 meatus. This canal is terminated by a plate of bone named the lamina cribrosa, 

 from the numerous minute apertures which it presents for the divisions of the 

 auditory nerve, while in its upper and fore part is the beginning of the canal called 

 aqueduct of Fallopius, which transmits the facial nerve. The aqueduct takes a some- 

 what circuitous course through the petrous bone, passing outwards and backwards 

 over the labyrinth of the ear, and then downwards to terminate at the stylo-mastoid 

 foramen. 



The anterior or upper surface looks upwards and forwards, and forms part of the 

 middle fossa of the base of the skull. A depression near the apex marks the 

 position of the Gasserian ganglion. A narrow groove runs obliquely backwards and 

 outwards to a foramen named the hiatus Fallopii, which leads to the aqueduct of 

 Fallopius, and transmits the large superficial petrosal nerve. Farther back is a 

 rounded eminence, indicating the situation of the superior semicircular canal. 



The line of separation of this surface of the petrous from the internal surface of the 

 squamous is marked by a narrow jjetro-squam ous Jissure, commencing anteriorly at the retiring 

 angle between the two portions, and generally to be traced less distinctly to the posterior 

 border of the bone. The portion of bone between this fissure externally and the eminence of the 

 superior semicircular canal and the hiatus Fallopii internally is a thin lamina, often perforated, 

 which roofs in the tympanum and the common canal of the Eustachian tube and tensor 

 tympani muscle, and is known as the teg-men tympani. 



The superior border is grooved for the superior petrosal sinus. The anterior 

 border is very short, and forms at its junction with the squamous part an angle in 

 which is situated the orifice of the Eustachian canal, the osseous portion of a tube 

 of the same name, which leads from the pharynx to the tympanum ; and above 

 this, par -ially separated from it by a thin lamella, the cochleariform process, is a 

 small passage which lodges the tensor tympani muscle. The posterior or inferior 

 border internal to the jugular fossa articulates with the basilar process of the 

 occipital bone, and forms with that the groove for the inferior petrosal sinus. 



Small foramina, &c. The opening of the aqueduct of the vestilule is a narrow fissure, 

 covered py a depressed scale of bone, and situated on the posterior surface of the petrous portion, 

 about four lines outside the internal auditory meatus ; that of the aqueduct of the coclilea is a 

 small foramen, beginning in a three-sided wider depression in the inferior margin, directly 



