Fig. 240. Diagram to show Exposure of the Semicircular Canals. The ‘black ring’ shows the area at which the semicircular canals and vestibule may be opened.
The next step is to remove the eminence of the horizontal semicircular canal. This is best done by means of a small gouge and mallet. Some prefer a burr, specially constructed to cut vertically; others a chisel. I prefer a fine gouge. As the facial canal runs along the lower anterior portion of the external semicircular canal, the gouge should be directed in a backward direction in removal of the outer wall of the latter, so as to cut away from the facial canal.
The surgeon should be content to remove the bone piecemeal, as, owing to its brittleness, it is very apt to splinter, or the point of the gouge itself may slip and so injure the facial nerve.
After an opening has been made into the canal, it should be enlarged by following the canal forward until its ampulla is reached. After this has been done, a fine probe, bent at a right angle (Schwartze’s seeker will do very well), is passed into the opening, and the limits of the vestibule made out as far as possible. The bone is then removed in an upward direction until the ampulla of the superior canal is reached. The opening may then be extended backwards so as to remove the outer wall of the vestibule, that is, the portion of bone which lies between the ampullæ of the superior and external canals.
If the bone be sclerosed, so that it is impossible to find the superior and posterior canals, then, after opening the exposed portion of the external semicircular canal, the bone should be chiselled away at the area marked out in Fig. 240. By this means the vestibule will certainly be reached, and from this point its opening can be extended in any given direction. A sufficient opening should be made so that the inner portion of the vestibule can be seen (Fig. 241). During each step of the operation a clear view must be obtained.
Not infrequently the facial nerve is exposed or pressed upon in chipping away the outer wall of the external semicircular canal, as will be shown by sudden twitchings of the face. If the surgeon be careful, and works in a direction away from the nerve, it should not be injured. If possible, the outer margin of the horizontal semicircular canal, together with the Fallopian canal, should be left intact as a bridge crossing the vestibule. If necessary, the external and superior canals can be removed in their entirety. A fine probe is inserted into the lumen of the canal so as to tell its direction, and its outer wall is then burred away. For this particular purpose a burr should be used as soon as the surgeon has got beyond the region of the facial nerve. After a view of the interior of the vestibule has been obtained, the ampullary nerves may be destroyed by means of the curette or with pure carbolic acid at the end of a probe. Removal of the posterior canal is best effected by opening it just behind the external semicircular canal and following it out in an upward direction until it meets the superior, and then downwards until it enters the vestibule. This extensive operation is one of extreme difficulty and seldom necessary.
Posterior to the semicircular canals: Neumann’s method. Neumann enters the vestibule posteriorly. The bone forming the inner wall of the antrum is removed by means of bone forceps or gouge and mallet until the posterior semicircular canal is opened. By this means the posterior surface of the petrous bone can be exposed as far inwards as the internal auditory meatus.
Fig. 241. Operation upon the Labyrinth. To show the opening into the vestibule above the facial nerve with partial or complete removal of the semicircular canals. The arrow passes behind the facial canal between the vestibule and the fenestra ovalis.