44 OSTEOLOGY, OR ANATOMY OF OSSEOUS SYSTEM 



and eminence of the superior semicircular canal internally 

 is a thin lamina which roofs in the tympanum and a common 

 canal for the Eustachian tube and tensor tympani muscle; 

 it is the legmen tympani, a process of the petrous. 



The posterointernal surface is in the posterior fossa of the 

 skull, and continuous with the inner surface of the mastoid. 

 Near the centre, but nearer the upper than the lower border, 

 is a large orifice, the porus acusticus internus, leading into a 

 canal } inch (6 mm.) long, which is the internal auditory meatus; 

 this is terminated by the lamina cribrosa. A transverse ridge, 

 cristafalciformis, separates a small superior from a large inferior 

 fossa. A faint perpendicular crest divides these into four 

 fossae. The facial nerve enters the aqueduct of Fallopius 

 in the upper anterior fossa; the area cribrosa superior is the 

 perforated part of the upper posterior fossa for the auditory 

 nerves going to the utricle, superior, and external auditory 

 canals; below this is the area crihroxa media, conveying an 

 auditory branch to the saccule; also the foramen singulare, 

 for a branch to the posterior auditory canal; in the lower 

 anterior fossa is the tractus spiralis faraminosus, for the cochlear 

 division of the auditory nerve, ending at the foramen cenfrale 

 cochleae. 



Behind the auditory meatus is a small slit, the opening of 

 the aqueduct of the rextihule, transmitting a small artery and 

 vein and lodging a process of dura mater which encloses the 

 saccus endolymphcdicus; above and between these is a depres- 

 sion or fissure, the subarcuate fossa, which extends into the 

 arch of the superior semicircular canal and represents the 

 floccular fossa of animals. 



The posterocxternal surface forms part of the base of the 

 skull. Beginning at the apex is first a quadrilateral surface 

 for the origin of the levator palati and tensor tympani muscles, 

 the lower aperture of the carotid canal, which is first vertical 

 and then horizontal; vertically beneath the internal auditory 

 meatus is the three-sided opening of the aqueduct of the cochlea, 

 which in early life transmits a vein; next behind is the jugular 

 fossa, which forms the jugular foramen when opposite the 

 jugular notch of the occipital. 



In front of the bony ridge, between the carotid canal and 

 jugular fossa, is a small foramen for Jacobson's nerve (from 

 the glossopharyngeal) to the tympanic plexus; this foramen 



