Bill’s bar and Internal acoustic canal

Bill’s bar

Bill bar is a bony anatomical landmark that divides the advanced compartment of the internal acoustic meatus into an anterior and posterior compartment. Anterior to Bill bar, inside the anterior superior quadrant, are the facial nerve (CN VII) and nervus intermedius, and posterior to it, in the posterior advanced quadrant is the advanced division of the vestibular nerve. The upper quadrants are separated from the lower quadrants by means of the falciform crest.
A recent have a look at evaluating the capacity of a medical 7 T magnet to differentiate the finer internal ear anatomy, in contrast with a three T magnet, found that Bill bar was on Bill bar is a bony anatomical landmark that divides the advanced compartment of the internal acoustic meatus into an anterior and posterior compartment. Anterior to Bill bar, inside the anterior superior quadrant, are the facial nerve (CN VII) and nervus intermedius, and posterior to it, in the posterior advanced quadrant is the advanced division of the vestibular nerve. The upper quadrants are separated from the lower quadrants by means of the falciform crest.
A recent have a look at evaluating the capacity of a medical 7 T magnet to differentiate the finer internal ear anatomy, in contrast with a three T magnet, found that Bill bar was on occasion visible at both discipline strengths 4. It has usually not been feasible to peer on HRCT of the temporal bone.

Internal acoustic canal

The internal acoustic canal (IAC), also known as the inner auditory canal or meatus (IAM), is a bony canal in the petrous part of the temporal bone that transmits nerves and vessels from in the posterior cranial fossa to the auditory and vestibular apparatus.

Gross anatomy

The opening of the IAM, the porus acusticus internus, is located within the cranial cavity, near the posterior surface of the temporal bone. The margins of the opening are smooth and rounded, and the canal is short (1 cm), running laterally to the bone. The canal narrows laterally, and the lateral boundary is the fundus, where the canal splits into three distinct openings, one of which is the facial nerve canal.

Contents

Contents of the IAM include:

  • facial nerve
  • vestibulocochlear nerve
  • vestibular ganglion
  • labyrinthine artery (usually a branch of the AICA or basilar artery)
Nerves

There are five nerves that run through the IAM:

  • nervus intermedius (sensory component of CN VII)
  • facial motor root (motor component of CN VII)
  • cochlear nerve (component of CN VIII)
  • inferior vestibular nerve (component of CN VIII)
  • superior vestibular nerve (component of CN VIII)

Their position is most constant in the lateral portion of the meatus which is anatomically divided by the falciform crescent. This horizontal ridge divides the canal into superior and inferior portions:

  • superior: facial nerve and superior vestibular nerve (SVN); the facial nerve is anterior to the SVN and is separated from it laterally by Bill’s bar, a vertical ridge of bone
  • inferior: cochlear nerve and inferior vestibular nerve (IVN); the cochlear nerve is situated anteriorly
Ganglion

In addition to the three nerves which enter it, it also carries the vestibular ganglion (ganglion of Scarpa). From here three bundles emerge: advanced and inferior department of the vestibular nerve and the nerve from the posterior semicircular canal.

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