A manual of otology for students and practitioners . fineprobe will now pass through either opening into thevestibule. The lower one enters through the ampullatedextremity of the canal into the lower part of the vestibulewhile the upper enters tlii^ough the crus commune to theupper and back part of this cavity. These two openingsare therefore the guides to the vestibule. Opening the Vestibule.—As the removal of the boneshavings from the posterior sm^face of the petrous pyramidis continued a third opening develops between the othertwo. This is the posterior end of the horizontal canal 340 SUPPU
A manual of otology for students and practitioners . fineprobe will now pass through either opening into thevestibule. The lower one enters through the ampullatedextremity of the canal into the lower part of the vestibulewhile the upper enters tlii^ough the crus commune to theupper and back part of this cavity. These two openingsare therefore the guides to the vestibule. Opening the Vestibule.—As the removal of the boneshavings from the posterior sm^face of the petrous pyramidis continued a third opening develops between the othertwo. This is the posterior end of the horizontal canal 340 SUPPURATIVE DISEASES OF LABYRINTH and furnishes an additional guide to the vestibule (). This cavity is soon opened. When it is freelyexposed one may be working very near the descendinglimb of the facial nerve, the location of which may bedetermined by estimating how much of the posteriorend of the horizontal canal has been removed, as the nerveturns downward about in the middle of this canal or atthe junction of its middle and posterior Fig. 113.—Neumanns labyrinth operation, showing the two openingsof the posterior semicircular canal and more anteriorly (to the left)the posterior end of the horizontal semicircular canal. Opening the Internal Auditory Meatus.—The next step isthe removal of the triangular process of bone which Hesbetween the vestibule and the internal auditory meatus ends at the vestibule, as the vestibularnerves occupy its posterior part and some of them passdirectly into the vestibule, which thus becomes theguide to the internal end of the meatus. With the curette,small gouges and chisels the triangular piece of bone isremoved, taking care that the diu*a is separated from thebone. Neumann advises that the dura in the middle NEUMANNS OPERATION 341 fossa be exposed but the author prefers to leave the mnertable in this location until the end of the operation fortwo reasons: (1) There is more danger of wounding thesuperior petrosal sin
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