THE TEMPORAL BOXE. 



37 



muscle ; and internal to that is the slight occipital groove, for the occipital 

 artery. The internal surface of the mastoid portion is marked by a deep 

 sigmoid depression, descending in the angle between it and the petrous 

 portion, which is part of the groove of the lateral sinus. A passage for a 

 vein, of very variable size, the mastoid foramen, usually pierces ths bone 

 near its posterior margin, and opens into the groove. 



Fig. 34. 



Fig. 34. THE RIGHT TEMPORAL BONE FROM 

 BELOW. | 



The indications where marked are the same 

 as in the preceding figures. 14, is at the apex 

 of the petrous bone in the upper opening of the 

 carotid canal; 17, aqueduct of the cochlea; 



21, lower rough surface of the petrous bone ; 



22, the lower opening of the carotid canal ; 



23, the small foramen of Jacobson's nerve ; 



24, the jugular depression, and within it, 25, 

 the foramen of Arnold's nerve ; 26, stylo-mas- 

 toid foramen the figure is placed in the ante- 

 rior part of the digastric groove ; 27, groove of 

 the occipital artery ; 28, place of the anterior 

 opening of the osseous Eustachian canal. 



The PETROUS PORTION is named from 

 its hardness. It contains the organ of 

 hearing. It forms a three-sided pyra- 

 mid, with its base directed outwards, 

 one surface looking downwards, and 

 the other two turned towards the interior of the skull. 



Inferior surface, base and apex. At the base is the aperture of the ear. 

 It forms a short canal, the meatus auditorius externus, directed inwards 

 and a little forwards, narrower in the middle than at its extremities, and 

 leading into the cavity of the tympanum. It is bounded superiorly by 

 the posterior root of the zygoma, and posteriorly in the remainder of its 

 circumference chiefly by the external auditory process, a curved uneven 

 border, to which the cartilage of the ear is attached. This process is the 

 thickened outer extremity of the tympanic plate, a lamina one surface of 

 which forms the anterior wall of the external auditory meatus and the tym- 

 panum, while the other looks forwards and downwards. The anterior margin 

 of the tympanic plate is separated from the glenoid fossa by the fissure ofGlaser, 

 which communicates with the tympanum, while its posterior margin descends 

 as a sharp edge, the vaginal process, which partly surrounds the front of the 

 styloid 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. A smooth rounded and deep depression, 

 the jugular fossa, lies internal to the styloid process ; it is close to the pos- 

 terior margin of the bone, and completes with the jugular notch of the occipital 

 bone the foramen lacerum posterius. In front and a little to the inside of 

 the jugular fossa is the carotid foramen, the inferior extremity of the carotid 

 canal ; and internal to the carotid foramen is a rough surface which gives 

 attachment to the levator palati muscle, and is continued into the rough 

 inner extremity, or apex of the petrous bone. The carotid canal ascends, 

 at first perpendicularly, then turns horizontally forwards and inwards, and 



