HEAD AND NECK 25
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.
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.
Radiology of the lacrimal gland Dacrocystography
The middle ear
The canaliculi may be cannulated and injected with 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 gamma camera.
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 with 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
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.
Fig. 1. 31 Ear: coronal section showing outer, middle and inner ear.
