It commences at the introitus of facial canal at the distal end of the internal auditory meatus.

The horizontal part is further subdivided into two crura: the proximal/medial[4] anterolaterally[5] directed medial crus (or labyrinthine segment[5]), and the distal/lateral[4] posterolaterally[5] directed lateral crus (or tympanic segment[5]); the two crura meet at a sharp angle at the genu of facial canal (geniculum canalis facialis[6]) where the geniculate ganglion is situated (at the genu, the greater petrosal nerve leaves the facial canal through the hiatus of the facial canal).

[4] The lateral crus of horizontal part ends by turning sharply inferior-ward, commencing the distal-most descending part (or mastoid segment[5]) of facial canal which passes vertically inferior-ward, ending distally at the stylomastoid foramen.

[citation needed] The tympanic segment is closely related to the posterior and medial walls of the tympanic cavity; it passes superior to the oval window and inferior to the lateral semicircular canal.

This may lead to the facial nerve being split into 2 or 3 fibres, or it may be poorly formed or congenitally absent on one side.