The pharynx and related spaces

Within the bony labyrinth is the membranous labyrinth, which comprises the utricle which is connected to the semicircular canals, and the saccule which is connected w i th the cochlear canal. A utriculosaccular duct connects these two. The endolymphatic duct arises from this connecting duct and passes in the vestibular aqueduct to end in a blind dilatation, the endolymphatic sac. The membranous labyrinth is filled w i th endolymph and surrounded by perilymph. The perilymph is continuous w i th CSF in the subarachnoid space at the vestibular aqueduct (Fig. 1. 32).

The cochlea has between 21/2 and 23/4 turns and its apex points anterolaterally so that the axis of the cochlea is perpendicular to the axis of the petrous bone. The bony cochlea has a central modiolus from which a shelf-like spiral lamina projects.

The internal auditory meatus (Fig. 1. 33) This bony canal is about 1 cm long and transmits the seventh and eighth cranial nerves from the posterior cranial fossa. Its lateral extent is separated from the inner ear by a perforated plate of bone. A crest on this bone, the crista falciformis, divides the canal into upper and lower compartments. The upper compartment contains the facial nerve and the superior vestibular branch of the eighth cranial nerve, and the lower compartment contains the acoustic and inferior vestibular branches of the eighth cranial nerve. The vestibular and acoustic nerves pass through the perforated plate of bone into the inner ear. The facial nerve turns anteriorly through the anterior wall of the lateral part of the canal into its own bony canal. The first part of the facial nerve runs in the internal auditory meatus. The second part runs anteriorly from this in its bony canal, then curves laterally and posteriorly around the cochlea to the anterior part of the medial wall of the middle-ear cavity. This U-bend around the cochlea is known as the genu of the nerve. The third and fourth parts have been described in the section on the middle ear.

The posterior lip of the medial end of the internal auditory meatus is called the porus acousticus and is normally sharply defined. It may be eroded by pathology in this region.

Development of the ear

The external and middle ear develop from the first and second branchial arches. The inner ear develops from the otic capsule.

Radiology of the middle and inner ear

Plain films

The internal acoustic meati and parts of the bony labyrinth of the inner ear may be identified on a straight OF skull projection. These images and tomography have been superceded by CT and MRI for evaluation of the I A M.

Computed tomography (see Fig. 1. 33) Images may be obtained in both axial and coronal planes. The internal auditory meati are readily assessed (see Fig. 1. 33), and the extent of any pathology and its relationship to other important intracranial structures may be determined. High-resolution scanning of the temporal bone can image the cochlea, vestibule, semicircular canals, ossicles and bony canal of the facial nerve in axial or coronal planes.

On high cuts through the petrous bone the two limbs of the superior semicircular canal may be seen as two round bony defects.

At lower levels, the vestibule, w i th the horizontal semicircular canal projecting posteriorly in an arc, is seen. The entire horizontal semicircular canal may be seen as it lies in the plane of section. The internal auditory canal is seen medially, running laterally from the posterior cranial fossa to the cochlea. The vestibule is posterolateral. The posterior semicircular canal projects posteriorly from the vestibule. The malleus and incus are seen laterally in the epitympanic recess. The bony canal of the facial nerve passes from front to back, medial to the middle ear cavity, lateral to the vestibule and just below the level of the horizontal semicircular canal.

A slightly lower level passes through the external auditory canal and middle-ear cavity. The ossicles are seen traversing the middle-ear cavity. The basal turn of the cochlea is seen anteriorly. The lower part of the posterior semicircular canal and vestibule are posterolateral. The facial nerve in its bony canal may be seen in cross-section as a circular structure laterally running down to the stylomastoid foramen.

Magnetic resonance imaging (see Fig. 1. 32) MRI may also be used to study the contents of the temporal bone and has the advantage of being able to image in any plane. Coronal images demonstrate the contents of the internal auditory canal to advantage, showing the facial and cochlear nerves separated by a signal void caused by the falciform crest. The loop of the anterior inferior cerebellar artery as it coils near the I AM is often visible and should not be mistaken for pathology. The cochlea and vestibule may also be identified. These are fluid-filled and of high signal intensity (and thus easier to see) on T2-weighted images. Neither CT nor MRI can distinguish the membranous labyrinth from the fluid bony labyrinth in which it lies.

THE PHARYNX AND RELATED SPACES (Fig. 1. 34) The pharynx is a muscular tube extending from the base of the skull to the level of the cricoid cartilage, approximately C6, where it is continuous w i th the oesophagus. It lies

Fig. 1. 34 Pharynx: sagittal section: (a) diagram; (b) sagittal T1 MRI.

1. Soft palate 2. Adenoids 3. Middle turbinate 4. Inferior turbinate 5. Hard palate 6. Intrinsic muscle of tongue 7. Genioglossus 8. Mandible 9. Myelohyoid muscle 10. Hyoid bone 11. Epiglottis 12. Vocal cord 13. Thyroid cartilage 14. Nasopharynx 15. Oropharynx 16. Corniculate cartilage 17. Arytenoid cartilage 18. Cricoid cartilage