HEAD AND NECK 21
The sublingual gland (Fig. 1. 21) This small gland lies submucosally just anterior to the deep lobe of the submandibular gland and drains via several ducts (up to 20) directly into the floor of the mouth posterior to the opening of the submandibular gland. Some of its ducts may unite and join the submandibular gland. Radiology of the salivary glands Sialography (Fig. 1. 24) The ducts of the parotid and submandibular glands may be cannulated and injected with radio-opaque contrast to outline the ductal system. The ducts of the parotid gland arch around the mandible because of the way in which the gland is moulded to the adjacent structures. This is best seen on the AP view. The parotid duct is seen on the lateral view. The submandibular gland and duct system may be seen on the lateral projection. The ducts of the sublingual gland are not amenable to canalization. Cross-sectional imaging CT (see Figs 1. 23, 1. 35 and 1. 42) and MRI are of particular value for tumours of the glands, to assess involvement of surrounding structures. CT may be performed after sialography to improve visualization of the ducts. High-resolution MR images may actually demonstrate the facial nerve within the parotid gland. It is of slightly lower intensity than the surrounding gland on T1-weighted images. Ultrasound This may be performed through the skin or intraorally with high-frequency transducers. Nuclear imaging Because the salivary gland accumulates and secretes 99m technetium-99m ( Tc) used in nuclear imaging, this can be
used to image several glands at once without cannulating the ducts. Graphs of uptake and excretion of the agent by individual glands may be computed. THE ORBITAL CONTENTS (Fig. 1. 25) The orbit contains the lacrimal gland, the globe, the extraocular muscles (including levator palpebrae), the optic nerve and the ophthalmic vessels. The whole is embedded in fat. The orbit is limited anteriorly by the orbital septum. This is a thin layer of fascia that extends from the orbital rim to the superior and inferior tarsal plates, separating the orbital contents from the eyelids. A fascial layer, the periorbita, lines the bony cavity of the orbit and this is continuous with the dura mater of the brain through the superior orbital fissure and optic canal. The globe of the eye is composed of a transparent anterior part covered by the cornea, and an opaque posterior part covered by the sclera. These are joined at the corneoscleral junction, known as the limbus. The anterior and posterior extremities of the globe are known as the anterior and posterior poles. The midcoronal plane of the globe is the equator. A further layer of fascia, Tenon's capsule, covers the sclera from the limbus to the exit of the optic nerve from the eye. This facial layer fuses with the fascia of the extrinsic ocular muscles at their insertions. Anteriorly, a mucous membrane known as the conjunctiva covers the anterior aspect of the eye. It is reflected from the inner surface of the eyelids and fuses with the limbus. There are six extrinsic ocular muscles that insert into the sclera. The four rectus muscles, the superior, inferior, medial and lateral recti, arise from a common tendinous ring called the annulus of Zinn. This is attached to the lower border of the superior orbital fissure. These muscles insert into the corresponding aspects of the globe, anterior to its equator. The superior oblique arises from the sphenoid bone superomedial to the optic foramen. It passes through a tendinous ring, the trochlea, which is attached to the frontal bone in the superolateral part of the orbit, acting as a pulley. It then passes posteriorly to insert into the upper outer surface of the globe, posterior to the equator.
