130 



OSTEOLOGY. 



apex of the bone corresponds to a rough quadrilateral surface which forms the 

 floor of the carotid canal, and also serves for the attachment of the cartilaginous 

 part of the auditory tube as well as the origin of the levator veli palatini muscle ; 

 elsewhere it has attached to it the dense fibrous tissue which fills up the cleft 

 (petro-basilar fissure) between it and the basilar part of the occipital bone. 



The anterior surface of the petrous part bears the impress of the gyri of the 

 lower surface of the temporal lobe of the cerebrum, which rests upon it ; in addition, 

 there is a distinct but shallow depression (impressio trigemini) near the apex, 

 corresponding to the roof of the carotid canal ; in this is lodged the semilunar 

 ganglion on the sensory root of the trigeminal nerve. Lateral to the middle 

 of the anterior surface, and close to its superior border, is the elevation 

 (eminentia arcuata), more or less pronounced, which marks the position of the 

 superior semicircular canal, here developed within the substance of the bone. A little 



Tympanic antruin, the medial 



wall of which is related to the 



lateral semicircular canal 



'edial part of posterior wall of external 

 acoustic meatus left in situ 



Points to the recessus epitympanicus 



Mastoid air-cells 



Facial nerve 



Facial canal laid open, displaying the facial nerve within 



FlG. 139. 



Preparation to display the position and relations of the tympanic antrum. The greater part of the posterior 

 wall of the external acoustic meatus has been removed, leaving only a bridge of bone at its medial ex- 

 tremity ; under this a bristle is displayed, passing from the tympanic autrum through the iter to the 

 cavity of the tympanum. 



in front of this, and in line with the angle formed by the anterior border and the 

 squamous part, is the slit-like opening of the hiatus canalis facialis, within the 

 projecting lip of which two small orifices can usually be seen. These are the 

 openings of the canalis facialis; if a bristle is passed through the more medial 

 of the two openings it will be observed to pass into the bottom of the internal 

 acoustic meatus, if into the more lateral, it will pass through the facial canal, 

 and, provided the channel be clear, will appear on the inferior surface of the 

 bone at the stylo-mastoid foramen. Leading forwards and medially from the 

 hiatus towards the anterior border is a groove ; in this lies the greater superficial 

 petrosal nerve, which passes out of the hiatus. A small branch of the middle 

 meningeal artery also enters the bone here. A little lateral to the hiatus is 

 another small opening (apertura superior canalis tympanici), often difficult to see ; 

 from this a groove runs forwards which channels the upper surface of the roof of 

 the canal for the tensor tympani muscle. Through this foramen and along this 

 groove passes the lesser superficial petrosal nerve. Behind this, and in front of 

 the arcuate eminence, the bone is usually thin (as may be seen by holding it up to 

 the light falling through the external acoustic meatus), roofing in the cavity 



