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The vestibule

This is an oval chamber situated between the base of the internal auditory meatus and the inner wall of the tympanum, with which it communicates by way of the fenestra ovalis. Anteriorly the vestibule 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, which is finely perforated for the passage of nerve-twigs. This fovea is separated by a vertical ridge (the crista vestibuli) from the vestibular orifice of the aqueductus vestibuli (ductus endolymphaticus), which passes obliquely backwards to open on the posterior surface of the petrosal bone.

The roof contains an oval depression - the fovea hemielliptica. Anteriorly 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 about 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 limb makes a projection on the anterior surface of the bone. The ampulla is at the outer end; the inner end opens into the vestibule conjointly with 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 superior canal, and opens in common with it into the vestibule. The lower is the ampullated 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 ampulla is at the outer end near the fenestra ovalis.

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