Chapter 4

The Globe: a brief overview

Introduction

This chapter is deliberately brief to avoid repetition, as more detailed descriptions can be found in the individual chapters on each structure. Its aim is to enable you to see the interrelations of the various structures.

The globe or eyeball is situated in the bony socket or orbit (see p. 59), which affords it protection. Also in the socket are nerves, muscles, blood vessels and fat.

Anteriorly, the globe is also protected by the upper and lower eyelids (see p. 60), which contain muscles, secretory glands and eyelashes.

The lacrimal gland (see p. 79) sits in the upper outer aspect of the frontal bone of the orbit and produces tears. These tears drain into the lacrimal drainage system (see p. 80). This is composed of an upper and lower punctum situated on the inner aspects of the upper and lower lid margins, the upper and lower canaliculi and the lacrimal sac, which opens into the nasal duct.

There are six extra-ocular muscles (see p. 186), which move the eye in the direction of gaze. There are four recti muscles and two oblique muscles.

The conjunctiva lines the lids (the palpebral conjunctiva) and overlies the sclera (the bulbar conjunctiva), terminating at the cornea.

The globe (fig. 4.1) is approximately 2.5 cm in diameter by the age of three years. It has three layers:

(1)The outer protective layer comprises the sclera (see p. 105) for approximately its posterior five-sixths and the cornea (see p. 103) for its anterior one-sixth. The cornea is clear to allow light rays through and is highly sensitive. The sclera is composed of tough white fibrous tissue.

(2)The middle layer is the pigmented vascular uveal tract (see Chapter 9). The choroid forms approximately the posterior four-fifths and the ciliary body and iris the anterior one-fifth. The iris is a diaphragm allowing varying amounts of light to enter the eye through the pupil in its centre. The ciliary processes produce aqueous and the ciliary muscles control the shape of the lens for focusing. The choroidal blood vessels supply the underlying outer layers of the retina.

(3)The inner layer is formed by the retina (see p. 160) and is the nerve ending layer containing rods and cones, which receive the light

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stimulus that is sent via the optic nerve to the occipital cortex for interpretation.

Aqueous (see p. 130) is produced by the ciliary processes, which are part of the ciliary body, and flows into the posterior chamber, through the pupil, into the anterior chamber and drains through the trabecular meshwork and the canal of Schlemm in the angle of the anterior chamber. There is also some drainage via the uveo-scleral route. It nourishes the lens and cornea.

The anterior chamber (see p. 130) is the area between the cornea and the iris.

The posterior chamber (see p. 130) is the area between the posterior surface of the iris and the anterior surface of the lens.

The crystalline lens (see p. 148) is suspended by the suspensory ligaments (zonules) from the ciliary body and lies behind the iris. It is clear to allow light rays to pass through unhindered. It changes shape so light rays can be focused on the retina for near vision, a process known as accommodation.

Vitreous (see p. 164) is a clear gelatinous substance, which fills the posterior segment of the eye between the lens and the retina.

The nerve supply to the eye

The oculomotor or third cranial nerve supplies the:

•levator palpebral superioris muscle

•superior rectus muscle