THE TYMPAM'M 41 



The Fallopian canal. — This canal begins at the anterior anjrle of the .sujierior 

 fossa of tile internal auditory meatus, and passes directly outwards to the hiatus 

 Fallopii; it then turns abruptly backwards and forms a horizontal ridge on the 

 inner wall of the tym})anum, lying in the angle between it and the tegmen 

 tynipani. It passes immediately above the fenestra ovalis, and extends as far 

 backwards as the entrance to the mastoid antrum; here it c(jmes into contact with 

 the inferior aspect of the projection formed by the external semicircular canal. It 

 then turns vertically downwards, running in the angle between the internal an<l 

 posterior walls of the tym])anun) to terminate at the stylo-mastoid foramen. 



The' canal is traversed by the facial (seventh) nerve. Numerous openings exist 

 in the walls of this passage. At the hiatus, the greater and smaller superficial 

 petrosal nerves escape from, and a l)ranch from the middle meningeal artery enters, 

 the canal. In the vertical part of its course the cavity of the posterior pyramid 

 opens into it. There is also a small orifice l\v "which the auricular branch of the 

 vagus joins the facial, and near its termination the iter chordae posterius for the 

 chorda tympani nerve leads from it into the tympanum. 



The vestibule. — This is an oval chamber situated between the Itase of the 

 internal auditory meatus and the inner wall of the tympanuuK with which it com- 

 numicates by way of the fenestra ovalis. Anteriorly the vestil)ule leads into the 

 cochlea, and posteriorly it receives the extremities of the semicircular canals. It 

 measures about 3 mm. transversely, and is somewhat longer antero-posteriorly. 



Its inner wall presents at the anterior part a circular depression, the fovea 

 hemispherica, Avhich is finely perforated for the passage of nerve-twigs. This 

 fovea is separated by a vertical ridge (the crista vestibuli ) from the vestibular 



Fig. 49.— The Left Osseous LABYnivxir. (After Heule. From a cast. ) 



SUPERIOR SEMICIRCULAR CANAL 



EXTERNAL SEMICIRCULAR CANAL 



POSTERIOR SEMICIRCULAR CANAL 

 FENESTRA ROTUNDA FENESTRA OVALIS OR VESTIBULI 



orifice of the aqueductus vestibuli, which passes obliquely 1)ackwards to open on 

 the posterior surface of the petrosal bone. 



The roof contains an oval depression — the fovea hemielliptica. Antei'iorly 

 the vestibule leads into the cochlea. Posteriorly it receives the five openings of 

 the semicircular canals. 



The semicircular canals are three in number. Each forms al)OUt two-thirds 

 of a circle; they lie in different planes. One extremity of each canal is dilated to 

 form an ampulla. 



The superior canal lies transversely to the long axis of the petrosal, and is 

 nearly vertical; its highest liml) makes a projection on the anterior surface of the 

 bone. The ampulla is at the outer end; the inner end opens into the vestil)ule 

 conjointly witli the superior limb of the posterior canal. 



The posterior canal is nearly vertical and lies antero-posteriorly. It is the 

 longest of the three; its upper extremity joins the inner limb of the sujierior 

 canal, and ojiens in common with it into the vestibule. The lower is the ampul- 

 lated end. 



The external canal is placed horizontally and arches outwards; its external 

 limb forms a prominence in the mastoid antrum. This canal is the shortest; its 

 am])ulla is at the outer end near the fenestra ovalis. 



The cochlea. — This is a cone-shaped cavity lying with its base upon the intenial 

 audit(jry meatus, and the apex directed outwards. It measures about five milli- 

 metres in length, and the diameter of its base is about the same. The centre of this 

 cavity is occupied by a column of Ixjne — the modiolus — around wliidi a delicate 

 bony lamella appears to be wound. This lamella is the osseous spiral lamina, 



