The ear

The canaliculi drain into the lacrimal sac, which is situated in a bony groove in the medial wall of the orbit but outside the fascial plane, which limits the orbit proper. This drains via the nasolacrimal duct, which runs in its own bony canal to the inferior meatus of the nasal cavity. A mucosal fold at its inferior end, the plica lacrimalis, acts as a valve, the valve of Hasner, to prevent reflux into the duct.

Radiology of the lacrimal gland

Dacrocystography The canaliculi may be cannulated and injected w i th radioopaque contrast to outline the drainage system of the lacrimal apparatus. Patency of the duct can also be established by nuclear dacrocystography without cannulation of the duct. Drops containing radionuclide are dropped on to the conjunctiva and the path of the duct is imaged by g a m ma camera.

CT (Fig. 1. 30) and MRI These imaging techniques may be used to study the lacrimal gland and orbital contents. The bony canal of the nasolacrimal duct may be identified on axial and coronal CT images. THE EAR (Figs 1. 31-1. 33)

The external ear

The external ear consists of the pinna and the external auditory meatus. The external meatus is 3. 5 cm long and runs medially to the ear drum or tympanic membrane. The outer part of the canal is cartilaginous and the medial twothirds is bony. The entire canal is lined by skin.

The middle ear

The middle ear is a slit-like cavity housed in the petrous bone. It lies between the tympanic membrane laterally and the inner ear medially. It has an upper part, which is recessed superiorly into the petrous bone and is known as the epitympanic recess or attic, as it lies at a higher level than the tympanic membrane. The roof of the cavity is formed by a thin layer of bone called the tegmen tympani, separating it from the middle cranial fossa and temporal lobe of the brain. The attic communicates w i th the mastoid air cells through a narrow posterior opening called the aditus and antrum. This is important, as infection may spread from the middle ear to the mastoid air cells, which are related posteriorly to the sigmoid sinus and cerebellum in the posterior cranial fossa (see Fig. 1. 33). A tiny spur of

Fig. 1. 32 (a) Axial T2 MRI to show the internal auditory meatus and its contained nerves;

1. Cochlea 2. Vestibulocochlear nerve 3. Facial nerve 4. Basilar artery with flow artefact 5. Cerebellopontine angle cistern

(b) diagram of membranous labyrinth;

(c) membranous labyrinth as seen on 3D MRI.

1. Cochlear duct 2. Superior semicircular canal 3. Posterior semicircular canal 4. Lateral semicircular canal 5. Utricle 6. Endolymphatic duct 7. Utriculosaccular duct 8. Saccule 9. Internal auditory meatus

bone, the scutum, separates the external auditory canal and the antrum, where the tympanic membrane is attached. The presence or erosion of the scutum is a sensitive marker for erosion by middle-ear disease states, including cholesteatoma. The lower part of the middle ear contains the ossicles, and is continuous inferiorly w i th the eustachian tube, which opens into the lateral wall of the nasopharynx. This tube is 3. 5 cm long, bony at first, and cartilaginous in its lower portion.

The floor of the middle ear is a thin plate of bone separating the cavity from the bulb of the jugular vein.

The lateral wall of the cavity is the tympanic membrane and the ring of bone to which it is attached.

The medial wall of the middle ear also forms the lateral wall of the inner ear, and its middle-ear surface is shaped by the contents of the inner ear. The lateral semicircular canal causes a prominence in the wall superiorly. Below this is a bulge caused by the cochlea called the promontory. The oval window, into which the base of the stapes inserts, is above and behind the promontory. The round window, covered by a membrane, is below and behind. The bony canal of the third part of the facial nerve also causes a prominence on the medial wall of the cavity. This runs from front to back, between the prominence of the lateral semicircular canal and the promontory, before turning down in the posterior wall of the cavity to emerge through the stylomastoid foramen (fourth part).

The ossicles traverse the middle ear cavity. The malleus has a handle that is attached to the tympanic membrane, and a rounded head that articulates w i th the body of the incus at the incudomallear joint. This joint is orientated superiorly and projects into the epitympanic recess. The incus has a long process that articulates w i th the head of the stapes, the base of which is firmly fixed in the oval window. The joints between the ossicles are synovial.

1. Cochlear turns 2. Internal auditory meatus 3. Vestibule 4. Posterior semicircular canal 5. Facial nerve in its bony canal

6. Malleus 7. Incus 8. Epitympanic recess 9. Mastoid air cells 10. Middle cranial fossa 11. Posterior cranial fossa 12. Lambdoid suture 13. Superior semicircular canal 14. Lateral semicircular canal 15. Oval window 16. Crista falciformis 17. Scutum 18. External auditory meatus

The inner ear

This is a membranous system of fluid-filled sacs concerned w i th hearing and balance. It is housed in a protective labyrinth of dense bone and lies in the petrous bone medial to the middle ear.

The bony labyrinth consists of a vestibule, which communicates posteriorly w i th the three semicircular canals, and anteriorly w i th the spiral cochlea. The bony covering of the lateral semicircular canal bulges into the medial wall of the middle-ear cavity and that of the superior semicircular canal forms the arcuate eminence on the superior aspect of the petrous bone. The vestibular aqueduct passes from the vestibule to open in the posterior fossa midway between the internal auditory meatus and the groove for the jugular vein. The cochlear duct extends from the cochlea to a slitlike opening in the posterior fossa inferior - though parallel - to the internal auditory meatus (IAM) (Fig. 1. 33).