214 



Anatomy of Skeleton 



posterior fossa and is nearly vertical in plane. It presents : a large and deep internal 

 auditory meatus for the entrance of the .seventh and eighth nerves, pars intermedia, 

 and auditory branch of basilar artery : about half an inch behind this the aquceductus 

 vestibuli, a narrow slit containing the blind terminal part of the ductus endolymphaticus. 



This is the remains of the stalk of the otocyst, which (Fig. 174) is drawn out as a result of the 

 descent of the otic vesicle : the growth of the vesicle to form the inner ear, and of the surrounding 

 petrous to enclose it, leads to the undeveloped " duct " being embedded in the inner aspect of the 

 bone, and between it and the dura mater. 



Above and between the meatus and aqueduct is the subarcuate fossa, another slit- 

 like depression, containing a fold of dura mater, the remains of a larger fossa present 

 in the young skull and described later : below the meatus, close to the lower border 

 of the surface, the opening of the aquceductus cochlea, immediately above the fossette 

 that lodges the petrous ganglion of the glosso-pharyngeal nerve (Fig. 175). 



This foramen transmits a minute vein to the inferior petrosal sinus, also the " ductus cochleae " 



from the scala tympani of the bony 

 labyrinth to the subarachnoid space ; 

 the dura mater turns into the foramen 

 to make a tunnel for the passage of the 

 ductus. 



Along the lower border of 

 the inner surface, in front of the 

 aquasductus cochleaa, there runs a 

 half-groove for the inferior petrosal 

 sinus (see Fig. 175). The groove 

 for the lateral sinus turns down 

 behind the projecting part of the 

 bone, on the mastoid portion, 

 and this part of the bone is 

 curved inwards as it helps to 

 form the outer part of the back 

 wall of the posterior fossa of the 

 skull. 



The whole of the lower 

 margin of the inner surface articu- 

 lates with the occipital, except 

 where the jugular foramen occurs, 

 but a thin layer of cartilage is between them from the front end to the lateral sinus. 



The upper surface (Fig. 176) of the petrous looks also outwards and forwards. It 

 presents : a hollow near the apex, for the Gasserian or semilunar ganglion and trunk of 

 the fifth nerve : the eminentia arcuata marking the position of the superior semicircular 

 canal : in front of this, and rather outside it, the hiatus Fallopii, a foramen for the 

 exit of the great superficial petrosal nerve and entrance of the petrous branch of the 

 middle meningeal artery, and a groove leading downwards, inwards, and forwards 

 from the hiatus to the foramen lacerum for the reception of the nerve : outside the 

 hiatus another smaller opening and groove for the exit of the small superficial petrosal 

 nerve : another groove may usually be seen running from this opening to the neigh- 

 bouring border, for the nerve. Outside these openings, between them and the petro- 

 squamous suture, the surface is made by the Tegmen tympani, as a short examination 

 of the bone will show. 



FIR. 176. View of right temporal bone from above. 



