THE BONES OF THE HEAD 75 



and transmits the external superficial petrosal nerve. This latter, 

 however, with the nene, is inconstant. Another small groove 

 marks the upper wall of the canal for the tensor tympani muscle at 

 the petro-squamous angle, and leads backwards and outwards, 

 parallel with the preceding groove, to a minute foramen, situated a 

 little external to the hiatus FaUopii, called the accessory hiatus, 

 which transmits the small superficial petrosal ner\e. Behind and 

 external to the hiatus Fallopii is an elevation, called the eminentia 

 arcuata, which coincides wath the |X)sition of the superior semi- 

 circular canal of the internal ear. Between this eminence and the 

 hiatus Fallopii internally and the petro-squamous fissure externally 

 there is a plate of bone, called the tegmen tympani, which forms the 

 roof of the tympanic cavity and of the canal for the tensor tympani 

 muscle. 



The posterior surface, which looks backwards and inwards, forms 

 part of the posterior cranial fossa. It presents about its centre a 

 large opening, which leads into a short canal, called the internal 

 auditory meatus, for the passage of the facial and auditory nerves, 

 the pars intermedia of Wrisberg, and the auditory artery. At the 

 deep end of this meatus there is a perforated plate of bone, known 

 as the lamina cribrosa, which is divided into an upper and a 

 lower fossa by a transverse ridge, called the faleiform erest. The 

 upper fossa presents at its anterior part a special foramen which 

 leads into the aqueduct of Fallopius, and by this foramen the facial 

 nerve leaves the meatiis. The remainder of the upper fossa is known 

 as the superior vestibular area, and it is pierced by the nerves and 

 arteries destined for the utricle and the ampullae of the superior and 

 external semicircular canals. The lower fossa contains the cochlear 

 area, which is pierced by the cochlear ner\-es and arteries, the 

 inferior vestibular area for the ner\-es and arteries to the saccule, 

 and the foramen singulare for the ner\-es and arteries to the arapuUa 

 of the posterior semicircular canal. The aqueduct of Fallopius, 

 for the facial nerve, extends from the deep end of the internal 

 auditory meatus to the stylo-mastoid foramen, between which 

 points it takes a very circuitous course. It passes at first 

 horizontally outwards between the cochlea and vestibule to the 

 inner wall of the tympanum, then it bends sharply backwards, 

 lying above the fenestra ovalis, and finally, making another 

 abrupt bend, it descends in the angle between the inner and 

 posterior walls of the tympanum to the st\lo-mastoid foramen. 

 The hiatus Fallopii leads from the commencement of the aqueduct 

 to the superior surface of the petrous portion, and, as stated, 

 transmits the great superficial petrosal nerve. The aqueduct, as 

 it descends behind the tjTiipanum communicates with the canal 

 of the pyramid by an opening through which the nerve to the 

 stapedius reaches that muscle, and below the pjTamid it presents 

 another opening, called the iter chordae posterius, by which the 

 chorda tympani nerve passes into the t5anpanum. 



About \ inch external to the opening of the internal auditory 



