The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [311]
Figure 9.2 Horizontal section through the eyeball at the level of the optic nerve. Optic axis and axis of eyeball are included.
The globe – internally
The globe is divided into two chambers by the lens (see Figure 9.3): the anterior cavity, anterior to the lens, and the vitreous chamber, posterior to the lens. The anterior cavity is divided into the anterior chamber and the posterior chamber by the iris. It contains a clear, watery fluid called the aqueous humour. The vitreous chamber is filled with a jelly-like substance called the vitreous body or vitreous humour. Together, these two fluid-filled cavities help maintain the shape of the eyeball and the intraocular pressure (Tortora and Derrickson 2009).
Figure 9.3 The anterior cavity in front of the lens is incompletely divided into anterior chamber (anterior to iris) and posterior chamber (behind iris), which are continuous through the pupil. Aqueous humour, which fills the cavity, is formed by ciliary processes and reabsorbed into the venous blood by the canal of Schlemm.
The aqueous humour is continuously secreted by the ciliary process (a part of the ciliary body) located behind the iris. This fluid then permeates the posterior chamber, passing between the lens and the iris, and flows through the pupil into the anterior chamber. From the anterior chamber, the aqueous humour drains into the scleral venous sinus (canal of Schlemm) and is absorbed back into the bloodstream (Figure 9.2).
The aqueous humour is the principal source of nutrients and waste removal for the lens and cornea, as these structures have no direct blood supply. If the outflow of aqueous humour is blocked, excessive intraocular pressure may develop, leading to the disease process known as glaucoma. This excess pressure can cause degeneration of the retina, which may result in blindness (Lee 2006). The vitreous humour is a clear gelatinous substance which fills the vitreous chamber, which, unlike the aqueous humour, is produced during fetal development and is never replaced (Tortora and Derrickson 2009).
Extrinsic structures
Extrinsic structures of the eye protect the globe from external injury (Stollery et al. 2005).
Eyelashes: protect the eye from debris.
Eyebrows: prevent moisture, in particular sweat, from flowing into the eye.
Eyelids: the eyelid is made up of complex muscles for eye movement, glands for tear and oil production that serve as a cleansing mechanism against dirt and foreign objects, and sensitive nerves for defence (Watkinson and Seewoodhary 2007).
Lacrimal (tear) apparatus: tears are produced in the lacrimal glands located at the upper, outer edge of the eye. They are excreted onto the upper surface of the globe and wash over the ocular surface by the action of blinking. The function of tears is to clean, moisten and lubricate the ocular surface and eyelids. Tears also provide antisepsis as they contain an enzyme called lysozyme that is able to rupture the cell membranes of some bacteria, leading to their lysis and death (Forrester 2002). The tears collect in the nasal canthus (inner, medial aspect of the eye) from where they drain into the upper and lower lacrimal puncti which drain into the lacrimal sac. From here, the tears pass into the nasolacrimal duct and empty into the nasal cavity (Figure 9.4) (Marieb 2001, Tortora and Derrickson 2009).
Figure 9.4 Lacrimal apparatus.
Optic nerve
The optic nerve, which is responsible for vision (cranial nerve II), exits the eye to the side of the macula lutea at an area called the optic disc (see Figure 9.2). This area is sometimes referred to as the anatomical blind spot. The optic nerve passes from the orbit through the optic foramen and into the brain. The two separate optic nerves meet at the optic chiasma and some optic nerve fibres cross over here to the opposite side of the brain. The nerves then continue along the optic