Eye Anatomy

A Brief Anatomy of the Eye
Gray's Anatomy 39th

The eyeball, the peripheral organ of vision, is situated in a skeletal cavity, the orbit, the walls of which help to protect it from injury. The orbit also has a more fundamental role in the visual process itself, in providing a rigid support and direction to the eye and in forming the sites of attachment for its external muscles. This setting permits the accurate positioning of the visual axis under neuromuscular control, and determines the spatial relationship between the two eyes - essential for binocular vision and conjugate eye movements.

The eyeball is embedded in orbital fat, separated from it by a thin fascial sheath. It is composed of the segments of two spheres of different radii. The anterior segment, part of the smaller sphere, is transparent and forms c.7% of the surface of the whole globe. It is more prominent than the posterior segment, which is part of a larger sphere and opaque, and forms the remainder of the globe. The anterior segment is bounded by the cornea and the lens, and is incompletely subdivided into anterior and posterior chambers by the iris. These chambers are continuous through the pupil. The anterior chamber is slightly overlapped by the sclera peripherally. The angle between the iris and cornea therefore forms an annulus of greater diameter than the limbus, the junction between the sclera and cornea. The difference between these two varies from 1 to 2 mm, the angle being deeper above and below than at the sides of the eyeball. The posterior chamber lies between the posterior surface of the iris and the anterior aspect of the lens and its supporting ligament, the zonule, and is triangular in section. The apex of the triangle is the point where the iris touches the lens, and the base, or zonular region, extends among the collagenous bundles of the zonule, sometimes even into a retrozonular space between the zonule and the vitreous humour in the posterior segment of the eyeball. The posterior segment consists of the parts of the eye posterior to the zonule and lens.

The anterior pole is the centre of the anterior (corneal) curvature, and the posterior pole is the centre of its posterior (scleral) curvature; a line joining these two points forms the optic axis. (By the same convention, the eye has an equator, equidistant between the poles: any circumferential line joining the poles is a meridian.) The optic axes of the two eyes, in their primary position, are parallel and do not correspond with the orbital axes, which diverge anterolaterally at a marked angle to each other . The optic nerves follow the orbital axes and are therefore not parallel; each enters its eye c.3 mm medial (nasal) to the posterior pole. The ocular vertical diameter (23.5 mm) is rather less than the transverse and anteroposterior diameters (24 mm); the anteroposterior diameter at birth is c.17.5 mm and at puberty 20-21 mm; it may vary considerably in myopia (c.29 mm) and in hypermetropia (c.20 mm). In females all diameters are on average slightly less than in the male.

Figure 1 The organization of the eye, viewed from above. In this illustration the left eye and part of the lower eyelid are depicted in horizontal section and also cut away to show internal structure.

OCULAR FIBROUS TISSUE

The eye has three layers enclosing its contents. From the outer surface these are a fibrous layer, which consists of the sclera behind and the cornea in front; a vascular, pigmented layer which consists of (from behind forwards) the choroid, ciliary body and iris, collectively termed the uveal tract; and a neural layer, known as the retina.

The fibrous layer of the eyeball (Fig. 1) has an opaque posterior sclera and a transparent anterior cornea. Together these form the protective enclosing capsule of the eye, a semi-elastic structure which when made turgid by intraocular pressure, determines with great precision the optical geometry of the visual apparatus. The sclera also provides attachments for the extraocular muscles which rotate the eye, its smooth external surface rotating easily on the adjacent tissues of the orbit. The cornea admits light, refracts it towards a retinal focus, and plays an important role in the image-processing mechanism

OCULAR VASCULAR TUNIC

The vascular tunic, or uveal tract (Fig. 2), consists of the choroid, ciliary body and iris (Fig. 3), which collectively form a continuous structure. The choroid covers the internal scleral surface, and extends forwards to the ora serrata. The ciliary body continues forward from the choroid to the circumference of the iris, which is a circular diaphragm behind the cornea and in front of the lens. It presents an almost central aperture, the pupil

Figure 2 The vascular arrangements of the uveal tract. The long posterior ciliary arteries, one of which is visible (A), branch at the ora serrata (b) and feed the capillaries of the anterior part of the choroid. Short posterior ciliary arteries (C) divide rapidly to form the posterior part of the choriocapillaris. Anterior ciliary arteries (D) send recurrent branches to the choriocapillaris (e) and anterior rami to the major arterial circle (f). Branches from the circle extend into the iris (g) and to the limbus. Branches of the short posterior ciliary arteries (C) form an anastomotic circle (h) (of Zinn) round the optic disc, and twigs (i) from this join an arterial network on the optic nerve. The vorticose veins (J) are formed by the junctions (k) of suprachoroidal tributaries (l). Smaller tributaries are also shown (m, n). The veins draining the scleral venous sinus (o) join anterior ciliary veins and vorticose tributaries. (By permission from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.)

Figure 3. Composite view of the surfaces and internal strata of the iris. In a clockwise direction from above, the pupillary (A) and ciliary (B) zones are shown in successive segments. The first (brown iris) shows the anterior border layer and the openings of crypts (c). In the second segment (blue iris), the layer is much less prominent and the trabeculae of the stroma are more visible. The third segment shows the iridial vessels, including the major arterial circle (e) and the incomplete minor arterial circle (f). The fourth segment shows the muscle stratum, including the sphincter (g) and dilator (h) of the pupil. The everted 'pupillary ruff' of the epithelium on the posterior aspect of the iris (d) appears in all segments. The final segment, folded over for pictorial purposes, depicts this aspect of the iris, showing radial folds (i and j) and the adjoining ciliary processes (k). (By permission from Hogan MJ, Alvarado JA, Weddell JE 1971 Histology of the Human Eye. Philadelphia: WB Saunders.)

RETINA

The retina is the sensory neural layer of the eyeball. It is a most complex structure and should be considered as a special area of the brain, from which it is derived by outgrowth from the diencephalon . It is dedicated to the detection and early analysis of visual information and is an integrated part of the much larger apparatus of visual analysis present in the thalamus, cortex and other areas of the central nervous system.

Layers of Retina

The retina is organized into layers or zones where distinctive components of its cells are clustered together or in register to form continuous strata. These layers extend uninterrupted throughout the photoreceptive retina except at the exit point of the optic nerve fibres at the optic disc, although certain layers are much reduced at the foveola where the photoreceptive elements predominate. The names given to the different layers reflect in part the components present within them, and also their position in the thickness of the retina. Conventionally, those structures furthest from the vitreous (i.e. towards the choroid) are designated as outer or external, and those towards the vitreous are inner or internal.

Customarily, ten retinal layers are distinguished (Fig. 4), beginning at the choroidal edge and passing towards the vitreous. These are: retinal pigment epithelium; layer of rods and cones (outer segments and inner segments); external limiting membrane; outer nuclear layer; outer plexiform layer (OPL); inner nuclear layer (INL); inner plexiform layer (IPL); ganglion cell layer; nerve fibre layer; internal limiting membrane. Some of these are subdivisible into substrata, and an innermost plexiform layer between layers 8 and 9 has also been demonstrated.


The composition of the different retinal layers is as follows:

Layer 1: Pigment epithelium

This is a simple low cuboidal epithelium which forms the back of the retina, and, therefore forms the boundary with the choroid, from which it is separated by a thick composite basal lamina.

Layer 2: Rod and cone cell processes

This contains the photoreceptive outer segments and the outer part of the inner segments of rod and cone cells.

Layer 3: External limiting membrane

This layer appears as a distinct line by light microscopy. It consists of a zone of intercellular junctions of the zonula adherens type (p. 7) between the processes of radial glial cells and photoreceptor processes.

Layer 4: Outer nuclear layer

This consists of several tiers of rod and cone cell bodies and their nuclei, the cone nuclei lying outermost. Mingled with these are the outer and inner fibres from the same cell bodies, directed outward to the bases of inner segments, and inwards towards the outer plexiform layer.

Layer 5: Outer plexiform layer

This is a region of complex synaptic arrangements between the processes of the cells whose cell bodies lie in the adjacent layers. The outer plexiform layer contains the synaptic processes of rod and cone cells, bipolar cells, horizontal cells, and some interplexiform cells (which in this account are grouped with the amacrines).

Layer 6: Inner nuclear layer

This is composed of three nuclear strata. Horizontal cell nuclei form the outermost zone, then in sequence inwards, the nuclei and cell bodies of bipolar cells, radial glial cells, and the outer set of amacrine cells, including the interplexiform cells whose dendrites cross this layer.

Layer 7: Inner plexiform layer

This is divisible into three layers depending on the types of contact occurring. The outer or 'OFF' layer contains synapses between 'OFF' bipolar cells, ganglion cells and some amacrines; a middle or 'ON' layer contains synapses between the axons of 'ON' bipolars and the dendrites of ganglion cells and displaced amacrines; and an inner 'rod' layer contains synapses between rod bipolars and displaced amacrines. (Refer to Wässle & Boycott 1991 for an explanation of the 'OFF' and 'ON' cell designations.)

Layer 8: Ganglion cell layer
This layer contains the nuclei of the displaced amacrine cells. Its inner regions consist of the cell bodies, nuclei and initial segments of retinal ganglion cells of various classes.

Layer 9: Nerve fibre layer

This contains the unmyelinated axons of retinal ganglion cells. It forms a zone of variable thickness over the inner retinal surface, and is the only component of the retina at the point where the fibres pass into the nerve at the optic disc. The inner aspect of this layer contains the nuclei and processes of astrocytes which, together with radial glial cells, ensheath the nerve fibres. Between the nerve fibre layer and the ganglion cells there is another narrow innermost plexiform layer where neuronal processes make synaptic contact with the axon hillocks and initial segments of ganglion cells.

Layer 10: Internal limiting membrane

This is a glial boundary between the retina and the vitreous body. It is formed by the end feet of radial glial cells and astrocytes, and is separated from the vitreous body by a basal lamina.

Figure 4 The layered arrangement of neuronal cell bodies in the retina and the interconnections of their processes in the intervening plexiform layers. Also shown are the two principal types of neuroglial cell in the retina; microglia are also present but not shown.

Optic disc

The optic disc is the region where retinal tissues meet the neural and glial elements of the optic nerve and the connective tissues of the sclera and meninges. It is the exit point for the optic nerve fibres, and a point of entry and exit for the retinal circulation. It is the only site where anastomoses occur with other arteries (the posterior ciliary arteries). It is visible, by ophthalmoscopy, and is a region of much clinical importance, since it is here that the central vessels can be inspected directly: the only vessels so accessible in the whole body. Oedema of the disc (papilloedema) may be the first sign of raised intracranial pressure, which is transmitted into the subarachnoid space around the optic nerve and compresses the central retinal vein where it crosses the space.

The optic disc is superomedial to the posterior pole of the eye, and so lies away from the visual axis. It is round or oval, usually c.1.6 mm in transverse diameter and 1.8 mm in vertical diameter, and its appearance is very variable (for details see Jonas et al 1988). In light-skinned subjects, the general retinal hue is a bright terracotta-red, with which the pale pink of the disc contrasts sharply; its central part is usually even paler and may be light grey. These differences are due in part to the degree of vascularization of the two regions, which is much less at the optic disc, and also to the total absence of choroidal or retinal pigment cells, since the retina is represented in the disc by little more than the internal limiting membrane. In subjects with strongly melanized skins, both retina and disc are darker . The optic disc does not project at all in many eyes, and rarely does it project sufficiently to justify the term papilla. It is usually a little elevated on its lateral side, where the papillomacular nerve fibres turn into the optic nerve. There is usually a slight depression where the retinal vessels traverse its centre.

RETINAL VASCULAR SUPPLY

The central retinal artery enters the optic nerve as a branch of the ophthalmic artery, c.1.2 cm behind the eyeball. It travels in the optic nerve to its head, where its fascicles traverse the lamina cribrosa. At this level, which is usually not visible to ophthalmoscopy, the central artery divides into two equal branches, superior and inferior. After a few millimetres, these divide into superior and inferior nasal, and superior and inferior temporal, branches. Each of these four supplies its own 'quadrant' of the retina, although each territory is much more than a quadrant, since the branches ramify as far as the ora serrata. Corresponding retinal veins unite to form the central retinal vein. However, the courses of the venous and arterial vessels do not correspond exactly, and arteries often cross veins, usually lying superficial to them. In severe hypertension the arteries may press on the veins and cause visible dilations distal to these crossings. Arterial pulsation is not visible by routine ophthalmoscopy without higher magnification.

The branching of the artery is usually dichotomous, and equal rami diverge at angles of 45-60°. Smaller branches may leave singly and at right angles. Arteries and veins ramify in the nerve fibre layer, near the internal limiting membrane, which accounts for their clarity when seen through an ophthalmoscope . Arterioles pass deeper into the retina and may penetrate to the internal nuclear lamina, from which venules return to larger superficial veins. The question of whether or not the dense capillary bed is diffusely organized or layered is unsettled. Some lamination has been identified, most noticeably at the interface between the inner nuclear and outer plexiform layers. The structure of the blood vessels resembles that of vessels elsewhere, except that the internal elastic lamina is absent from the arteries, and muscle cells may appear in their adventitia. Capillaries have a non-fenestrated endothelium.

OCULAR REFRACTIVE MEDIA

The components of the eye that transmit and refract light are the cornea, the aqueous humour, the lens and the vitreous body. Of these, only the refracting power of the lens can be varied.

Aqueous humour

To satisfy the requirements of vision the eye has its own circulatory system. Aqueous humour is secreted into the posterior chamber by the non-pigmented epithelium of the ciliary processes. It passes into the anterior chamber through the pupil and drains to the scleral venous sinus at the iridocorneal angle through the spaces of the trabecular tissue. It is responsible for maintaining the metabolism of the avascular transparent media, vitreous, lens and cornea, and it also maintains and regulates the relatively high intraocular pressure (c.17 mmHg), and hence the constancy of the ocular dimensions of the eyeball, via the balance between production and drainage. Depth of the anterior chamber may be assessed using slit-lamp biomicroscopy, and the filtration angle may be viewed directly by gonioscopy. Any interference with its drainage into the sinus increases intraocular pressure leading to the condition of glaucoma.

Lens

The lens is a transparent, encapsulated, biconvex body, which lies between the iris and the vitreous body. Posteriorly, the lens contacts the hyaloid fossa (p. 719) of the vitreous body. Anteriorly, it forms a ring of contact with the free border of the iris, but further away from the axis of the lens the gap between the two increases to form the posterior chamber of the eye (p. 708). The lens is encircled by the ciliary processes, and is attached to them by the zonular fibres which issue mainly from the pars plana of the ciliary body. Collectively, the fibres form the zonule which holds the lens in place and transmits the forces which stretch the lens (except in visual accommodation).

The lens has a characteristic shape. Its anterior convexity is less steep, and has a greater radius of curvature, than the posterior, which has a more parabolic shape. The central points of these surfaces are the anterior and posterior poles; a line connecting these is the axis of the lens. The marginal circumference of the lens is its equator. In fetuses the lens is nearly spherical, has a slight reddish tinge, and is soft, such that it breaks up on application of the slightest pressure. A hyaloid artery from the central retinal artery traverses the vitreous body to the posterior pole of the lens, whence its branches spread as a plexus. This covers the posterior surface and is continuous round the capsular circumference with the vessels of the pupillary membrane and iris.

In infants and adults the lens is avascular, colourless and transparent, but still quite soft in texture. In old age, the anterior surface becomes a little more curved, which pushes the iris forward slightly. It becomes less clear, with an amber tinge, and its nucleus is denser. In cataract, the lens gradually becomes opaque, causing blindness.

The dimensions of the lens are optically and clinically important, but they change with age as a consequence of continuous growth. Its equatorial diameter at birth is 6.5 mm, increasing rapidly at first, then more slowly to 9.0 mm at 15 years of age, and even more gradually to reach 9.5 mm in the ninth decade. Its axial dimension increases from 3.5-4.0 mm at birth to 4.75-5.0 mm at age 95. The radii of curvature reduce throughout life; the anterior surface shows the greater change as the lens thickens (Brown 1974). Average adult radii of the anterior and posterior surfaces are 10 mm and 6 mm respectively; the reduction during accommodation occurs mainly at the anterior surface.

Vitreous body

The vitreous body fills the vitreous chamber, and occupies about four-fifths of the eyeball. It is hollowed in front as a deep concavity, the hyaloid fossa, which is adapted to the lens. It is colourless, consisting of c.99% water, but not entirely structureless. At its perimeter it has a gel-like consistency (100-300μm thick) and is firmly attached to the surrounding structures of the eye; nearer the centre it has a more liquid zone in the form of long glycosaminoglycan chains, fills the whole vitreous. In addition, the peripheral gel or cortex contains a random loose network of type II collagen fibrils which are occasionally grouped into fibres. The cortex also contains scattered cells, the hyalocytes, which possess the characteristics of mononuclear phagocytes. They are responsible for the production of . Whilst they are normally in a resting state, they have the capacity to be actively phagocytic in inflammatory conditions. Hyalocytes are not present in the cortex bordering the lens. The liquid vitreous is absent at birth, appears first at 4 or 5 years, and increases to occupy half the vitreous space by the seventh decade. The cortex is most dense at the pars plana of the ciliary body adjacent to the ora serrata, where attachment is strongest, and this is often referred to as the base of the vitreous. Here the vitreous is thickened into a mass of radial (zonular) fibres which form the suspensory ligament of the lens

A narrow hyaloid canal runs from the optic nerve head to the central posterior surface of the lens. In the fetus this contains the hyaloid artery which normally disappears about 6 weeks before birth. It persists as a very delicate fibrous structure and is of no functional importance.