Structure of the Eyeball
The eyeball is a spheroidal structure in the orbital cavity that occupies most of the anterior part of the orbit (bony orbital socket of the skull). The eyeball is a three layered structure. The supporting outer fibrous layer, inner neural layer and the middle vascular layer constitute the trilaminar construction of the eyeball.
In addition, it is surrounded by a thin layer of loose connective tissue known as bulbar fascia which envelops from the optic nerve to the corneoscleral junction and also segregates the eyeball from the orbital fat. Anteriorly, this loose connective tissue also forms the bulbar conjunctiva. The three layers or coats of the eyeball are just as important as the entire eye, as they contain most of the structures that enable visual reflex, optical acuity, light refraction and vision. These structures along with their clinical relevance are described below.
Fibrous Layer of the Eyeball
This layer of the eyeball houses two optical structures of protective and mechanical importance. These structures are the sclera, which constitutes about 84% of the fibrous layer of the eyeball, and the cornea, which makes up the remaining 16% of the fibrous coat anteriorly. The sclera is the tough opaque part of the fibrous coat of the eyeball covering the posterior aspects of the eyeball. It constitutes the fibrous skeleton of the eyeball, providing shape and resistance as well as attachment for both the extrinsic or extraocular, and the intrinsic muscles of the eye. The anterior part of the sclera is visible through the transparent bulbar conjunctiva as the white of the eye. On the other hand, the cornea is the transparent part of the fibrous layer covering the anterior one-sixth (about the anterior 16%) of the eyeball. The two parts differ primarily in terms of the regularity of the arrangement of the collagen fibres of which they are composed and the degree of hydration of each. Generally, the collagen fibres in both parts are circularly or spherically arranged. There are few scleral vessels and nerves which terminate near the periphery. The cornea does not contain any blood vessels but it is richly innervated with numerous branches of the ophthalmic nerve. Scleral venous sinus (canal of Schlemm) lies near the internal surface of the sclera and filters the aqueous humor from the anterior chamber to the anterior ciliary veins.
Vascular Coat of the Eyeball
The vascular coat of the eyeball is the middle muscular layer. This layer of the eyeball is also known as the uvea or uveal tract. This layer consists of choroid, ciliary body, and iris. The iris, which lies on the anterior surface of the lens, is a thin contractile diaphragm with a central aperture, the pupil, for transmitting light. When a person is awake, the size of the pupil varies continually to regulate the amount of light entering the eye. Two involuntary muscles control the size of the pupil, sphincter pupillae innervated by parasympathetic fibres narrows the pupil and the sympathetically stimulated dilator pupillae muscle opens it. A second component of the uvea is the ciliary body, a muscular structure located behind the iris and that alters the shape of the lens during focusing and produces the aqueous humor that bathes the anterior chamber. This vascular ciliary body also connects the choroid with the circumference of the iris. The ciliary body provides attachment to the lens; contraction and relaxation of the smooth muscle of the ciliary body controls thickness (and therefore the focus) of the lens. More specifically, the folds on the internal surface of the ciliary body, the ciliary processes, secrete aqueous humor, which fills the anterior and posterior chambers of the eye. The anterior chamber of the eye is the space between the cornea anteriorly and the iris/pupil posteriorly. The posterior chamber of the eye is between the iris/pupil anteriorly and the lens and ciliary body posteriorly. Both ciliary body and iris receive sympathetic and parasympathetic fibres through short ciliary nerves. Long posterior and anterior ciliary arteries supply iris. Third is the choroid, this structure is a dark reddish brown layer between the sclera and the retina, forms the largest part of the vascular layer of the eyeball and lines most of the sclera. Within this pigmented and dense vascular bed, larger vessels of the vascular lamina are located externally (near the sclera). The finest vessels (the capillary lamina of the choroid or choriocapillaris, an extensive capillary bed) are innermost, adjacent to the avascular light-sensitive layer of the retina, and a source of oxygen and nutrients to this structure. The vessels of the choroid have autonomic vasomotor supply. Engorged with blood in life, this layer is responsible for the redness of the eye which reflects in flash photography. The choroid is continuous anteriorly with the ciliary body. The choroid attaches firmly to the pigment layer of the retina, but it can easily be stripped from the sclera.
Neural Layer of the Eyeball
The third and innermost coat of the eyeball is the neural layer of the eyeball. This layer constitutes what is referred to as the retina. The retina is a complex structure containing photoreceptors that convert the stimulus of light into nervous impulses. These receptors are of two types, rods and cones which are responsible for night and colour vision respectively, cones are best in bright light and are about seven million in each retina. The rods are far more numerous than the cones, and number more than one hundred million. The retina is also described as an extremely metabolically active layer of nerve tissue made up of millions of photo receptors and all of the structures needed to focus light onto it. Grossly, the retina consists of two functional parts with distinct locations: an optic part and a non-visual retina. The non-visual retina is an anterior continuation of the pigment cell layer and a layer of supporting cells over the ciliary body (ciliary part of the retina) and the posterior surface of the iris (iridial part of the retina), respectively. The optic part of the retina is sensitive to light rays and has two layers: a neural layer and pigment cell layer. The neural layer is light receptive. The pigment cell layer consist of a single layer of cells that reinforces the light-absorbing property of the choroid in reducing the scattering of light in the eyeball. The cones and rods of the outer neural layer of the retina receive nutrients from the capillary lamina of the choroid or choriocapillaris. It has the finest vessels of the inner surface of the choroid, against which the retina is pressed. A corresponding system of retinal veins unites to form the central vein of the retina. With exception to the cones and rods of the neural layer, the retina is supplied by the central artery of the retina, a branch of the ophthalmic artery. Furthermore, the functional optic part of the retina terminates anteriorly along the ora serrata (Latin word for serrated edges), an irregular border slightly posterior to the ciliary body. The ora serrata marks the anterior termination of the light-receptive part of the retina.
The fundus is the posterior part of the eyeball. It has a circular depressed area called the optic disc or optic papilla where the sensory fibres and vessels conveyed by the optic nerve enter the eyeball. Because it contains no photoreceptors, the optic disc is the blind spot. Just lateral to this spot is macula lutea (i.e., yellow spot in Latin), it is apparent only when the retina is examined with red-free light. The macula lutea is a small oval area of the retina with special photoreceptor cones that is specialized for visual acuity. At the centre of the macula lutea is a depression, the fovea centralis (Latin: central pit), the area of most acute vision. The fovea is approximately 1.5 mm in diameter; its centre, the foveola, does not have the capillary network visible elsewhere deep to the retina. An ophthalmoscope, which is a device for viewing the interior aspects of the eyeball through the pupil, does not normally capture the macula lutea when used to observe the eyeball. In addition to the eyeball coats described above, the cornea, lens, vitreous humor and aqueous humor components of the eyeball form what is referred to as “refractive media of the eyeball”. This media is described below.
Refractive Media of the Eyeball
Light rays are refracted as they travel through the interface between the air and anterior surface of the eyeball. As the light waves courses through the anterior chamber, iris, posterior chamber and lens to make their way to the retina, lightwaves pass through the refractive media of the eyeball, which is formed by the cornea, aqueous humor, lens, and vitreous humor. The cornea is largely responsible for refraction of light that enters the eye. It forms the circular area of the anterior part of the outer fibrous layer of the eyeball and is transparent, owing to the extremely regular arrangement of its collagen fibres and its dehydrated state. The cornea is sensitive to touch; its innervations is provided by the ophthalmic nerve. It is avascular. Its nourishment is derived from the capillary beds at its periphery, the aqueous humor, and lacrimal fluid. The latter also provides oxygen absorbed from the air. The lens is posterior to the iris and anterior to the vitreous humor of the vitreous body and is a transparent, biconvex structure enclosed in a capsule (the capsule of the lens or capsula lentis). The highly elastic capsule of the lens is anchored by the zonular fibres, also referred to as the suspensory ligament of the lens, to the ciliary body and encircled by the ciliary processes. Although most refraction is produced by the cornea, the convexity of the lens, particularly its anterior surface, constantly varies to fine-tune the focus of near or distant objects on the retina. The ciliary muscle in the ciliary body changes the shape of the lens; in this way the isolated unattached lens assumes a nearly spherical shape. Stretched within the circle of the relaxed ciliary body, the attachments around its periphery pull the lens relatively flat so that its refraction enables far vision. When parasympathetic stimulation causes the smooth muscle of the circular ciliary body to contract, the circle, like a sphincter, becomes smaller in size and the tension on the lens is reduced, allowing the lens to round up. The increased convexity makes its refraction suitable for near vision. In the absence of parasympathetic stimulation, the ciliary muscles relax again and the lens is pulled into its flatter, far-vision shape. The vitreous humor is a watery fluid enclosed in the meshes of the vitreous body, a transparent jelly-like in the posterior four fifths of the eyeball posterior to the lens (postremal or vitreous chamber, or posterior segment). In addition to transmitting light, the vitreous humor holds the retina in place and supports the lens. The aqueous humor is a clear watery solution which provides nutrients for the avascular cornea and lens. It fills the anterior and posterior chambers of the eyeball, but is produced in the posterior chamber by the ciliary processes of the ciliary body. After passing through the pupil into the anterior chamber, the aqueous humor drains into the scleral venous sinus (canal of Schlemm) at the iridocorneal angle. The humor is removed by the limbal plexus, a network of scleral vein close to the limbus, which drain in turn into tributaries of the vorticose and the anterior ciliary veins.