Ординатура / Офтальмология / Английские материалы / Lecture Notes on Ophthalmology_James, Chew, Bron_2003

2 Chapter 1: Anatomy

Between the cornea anteriorly and the lens and iris posteriorly lies the anterior chamber. Between the iris, the lens and the ciliary body lies the posterior chamber (which is distinct from the vitreous body). Both these chambers are filled with aqueous humour. Between the lens and the retina lies the vitreous body.

Anteriorly, the conjunctiva is reflected from the sclera onto the underside of the upper and lower eyelids. A connective tissue layer (Tenon’s capsule) separates the conjunctiva from the sclera and is prolonged backwards as a sheath around the rectus muscles.

ANATOMY OF THE EYE

Fig. 1.10The basic anatomy of the eye.

ORBIT (Fig.1.2)

The eye lies within the bony orbit whose structure is shown in Fig. 1.2.The orbit has the shape of a four-sided pyramid. At its posterior apex is the optic canal which transmits the optic nerve to the brain. The superior and inferior orbital fissures allow the passage of blood vessels and cranial

4Chapter 1: Anatomy

sympathetic supply is damaged (as in Horner’s syndrome) a slight ptosis results.

The margin of the eyelid is the site of the mucocutaneous junction. It contains the openings of the meibomian oil glands which are located in the tarsal plate. These secrete the lipid component of the tear film. Medially, on the upper and lower lids, two small puncta form the initial part of the lacrimal drainage system.

ANATOMY OF THE EYELIDS

Fig. 1.30The anatomy of the eyelids.

T H E L AC R I M A L D R A I N AG E S YS T E M (Fig.1.4)

Tears drain into the upper and lower puncta and then into the lacrimal sac via the upper and lower canaliculi.They form a common canaliculus before entering the lacrimal sac. The nasolacrimal duct passes from the sac to the nose. Failure of the distal part of the nasolacrimal duct to fully canalize at birth is the usual cause of a watering, sticky eye in a baby. Tear drainage is an active process. Each blink of the lids helps to pump tears through the system.

Detailed functional anatomy 5

LACRIMAL DRAINAGE SYSTEM

Upper canaliculus

Common canaliculus

Tear sac

Nasal mucosa

Nasolacrimal duct

Inferior turbinate

Inferior meatus

Puncta

Nasal cavity

Lower canaliculus

Fig. 1.40The major components of the lacrimal drainage system.

D E TA I L E D F U N C T I O N A L A N ATO M Y

The tear film

The tear film (10iµm thick) covers the external ocular surface and comprises three layers:

100a thin mucin layer in contact with the ocular surface and produced mainly by the conjunctival goblet cells;

200an aqueous layer produced by the lacrimal gland;

300a surface oil layer produced by the tarsal meibomian glands and delivered to the lid margins.

The functions of the tear film are as follows:

•00it provides a smooth air/tear interface for distortion free refraction of light at the cornea;

•00it provides oxygen anteriorly to the avascular cornea;

•00it removes debris and foreign particles from the ocular surface through the flow of tears;

•00it has antibacterial properties through the action of lysozyme, lactoferrin and the immunoglobulins, particularly secretory IgA.

Epithelium

Stroma

Endothelium

Fig. 1.50The structure of the cornea and precorneal tear film (schematic, not to scale).

8Chapter 1: Anatomy

•00phagocytoses the redundant external segments of the rods and cones;

•00facilitates the passage of nutrients and metabolites between the retina and choroid;

•00takes part in the regeneration of rhodopsin and cone opsin, the photoreceptor visual pigments recycling vitamin A;

•00melanin granules absorb scattered light.

The retina (Fig.1.7)

The retina:

•00Is a highly complex structure divided into ten separate layers comprising photoreceptors (rods and cones) and neurones, some of which (the ganglion cells) give rise to the optic nerve fibres.

•00Is responsible for converting light into electrical signals. The initial integration of these signals is also performed by the retina.

Cones are responsible for daylight vision. Subgroups of cones are responsive to different short, medium and long wavelengths (blue, green, red). They are concentrated at the fovea which is responsible for detailed vision such as reading fine print.

THE RETINA (a)

Vitreous

Inner limiting membrane

Nerve fibre layer

Ganglion cell layer

Inner plexiform layer

Inner nuclear layer

Outer plexiform layer

Receptor nuclear layer

External limiting membrane

Inner and outer segments of photoreceptors

RPE

Choroid

Fig. 1.70(a) The structure of the retina.

Detailed functional anatomy 9

RODS AND CONES (b)

Cone Rod

Outer plexiform layer

Outer

fibre

External limiting membrane

Inner segment

Ellipsoid

Cilium

Cilium

Outer segment

Discs

Retinal pigment epithelium

Fig. 1.70(b) The structure of the retinal rods and cones (schematic).

Rods are responsible for night vision.They are sensitive to light and do not signal wavelength information (colour). They form the large majority of photoreceptors in the remaining retina.

The vitreous

The vitreous:

•00Is a clear gel occupying two-thirds of the globe.

•00Is 98% water. The remainder consists of hyaluronic acid and a fine collagen network.There are few cells.

•00Is firmly attached anteriorly to the peripheral retina, pars plana and around the optic disc, and less firmly to the macula and retinal vessels.

•00Has a nutritive and supportive role.

Detachment of the vitreous from the retina, which commonly occurs in later life, increases traction on the points of firm attachment. This may occasionally lead to a peripheral retinal break, when the vitreous pulls away a piece of the underlying retina.