3. Structure of Eye

3.1 Anatomy of the Eye

3.1.1 External and Accessory Structures of Eye

• The adult eye is a sphere that measures about 1 inch (2.5 cm) in diameter.
• The accessory structures of the eye include the extrinsic eye muscles, eyelids, conjunctiva, and lacrimal apparatus.
• Anteriorly the eyes are protected by the eyelids, which meet at the medial and lateral corners of the eye, the medial and lateral commissure (canthus), respectively.
• The space between the eyelids in an open eye is called the palpebral fissure.
• Projecting from the border of each eyelid are the eyelashes.
• Modified sebaceous glands associated with the eyelid edges are the tarsal glands (These glands produce an oily secretion that lubricates the eye).
• Ciliary glands, modified sweat glands, lie between the eyelashes (cilium = eyelash).
• A delicate membrane, the conjunctiva, lines the eyelids and covers part of the outer surface of the eyeball.
• The lacrimal apparatus consists of the lacrimal gland and a number of ducts that drain the lacrimal secretions into the nasal cavity.
• The lacrimal glands are located above the lateral end of each eye. They continually release a dilute salt solution (tears) onto the anterior surface of the eyeball through several small ducts.
• The tears flush across the eyeball into the lacrimal canaliculi medially, then into the lacrimal sac, and finally into the nasolacrimal duct, which empties into the nasal cavity (Lacrimal secretion also contains mucus, antibodies, and lysozyme, an enzyme that destroys bacteria).
• Six extrinsic, or external, eye muscles are attached to the outer surface of each eye (these muscles produce gross eye movements and make it possible for the eyes to follow a moving object.).

3.2 Internal Structures of Eye

• The eye itself, commonly called the eyeball, is a hollow sphere.
• Its interior is filled with fluids called humors that help to maintain its shape.
• The lens, the main focusing apparatus of the eye, is supported upright within the eye cavity, dividing it into two chambers.

3.2.1 Its wall is composed of three layers.

• 1. Fibrous Layer (Outer)
  2. Vascular Layer (Middle)
  3. Sensory Layer (Inner)

1. Fibrous Layer

• The outermost layer, called the fibrous layer, consists of the protective sclera and the transparent cornea.
• The sclera, thick, glistening white connective tissue, is seen anteriorly as the “white of the eye.”
• Cornea is a “window” through which light enters the eye.
• In the back of the eye, the optic nerve and certain blood vessels pierce the sclera.

The cornea is the only tissue in the body that is transplanted from one person to another without the worry of rejection. Because the cornea has no blood vessels, it is beyond the reach of the immune system.

1. Vascular Layer

• The middle layer of the eyeball.
• Contains a dark pigment to prevent the scattering of incoming light rays.
• Two involuntary muscles make up the front part of the choroids:

Iris– the colored structure seen through the cornea,

Ciliary muscle– to which the lens is attached by a suspensory

Ligament.

• The pigmented iris has a rounded opening, the pupil, through which light passes.
• The ciliary muscles and suspensory ligaments, along with the structure of the lens itself, enable the lens to adjust shape to facilitate focusing, a phenomenon called accommodation.

1. Sensory Layer

• The innermost sensory layer of the eye is the delicate two-layered retina (which extends anteriorly only to the ciliary body).
• The outer pigmented layer of the retina is composed of pigmented cells.
• The transparent inner neural layer of the retina contains millions of receptor cells, the rods and cones, which are called photoreceptors.
• The photoreceptor cells are distributed over the entire retina, except where the optic nerve (composed of ganglion cell axons) leaves the eyeball; this site is called the optic disc, or blind spot.
• Cones are receptors that allow us to see the color under bright light conditions.
• Fovea centralis, a tiny pit that contains only cones (area of greatest visual acuity, or point of sharpest vision).

There are three varieties of cones. Each type is most sensitive to particular wavelengths of visible light.

One type responds most vigorously to blue light, another to green light. The third cone variety responds to a range including both green and red wavelengths of light.

3.2.2 Lens

• Light entering the eye is focused on the retina by the lens, a flexible biconvex crystal-like structure.
• The lens is held upright in the eye by a suspensory ligament, the ciliary zonule, attached to the ciliary body.
• The lens divides the eye into two chambers:
  • The anterior (aqueous) segment, anterior to the lens, contains a clear watery fluid called aqueous humor.
  • The posterior (vitreous) segment, posterior to the lens, is filled with a gel-like substance called either vitreous humor.
• Vitreous humor helps prevent the eyeball from collapsing inward by reinforcing it internally.
• Aqueous humor is similar to blood plasma and is continually secreted by a special area of the choroid. Like the vitreous humor, it helps maintain intraocular pressure, or the pressure inside the eye.

3.3 Visual Receptors & Visual Pigments

3.3.1 Visual Receptors

• Visual receptor cells are modified neurons.
• There are two types i.e. Rods and Cones.
• Rods, have long, thin projections at their ends, and provide black and white vision.
• Cones, have short, blunt projections, and provide color vision.
• Rods and cones are in a deep portion of the retina, closely associated with a layer of pigmented epithelium.
• Visual receptors are stimulated only when light reaches them.
• A light image focused on an area of the retina stimulates some receptors, and impulses travel from them to the brain.

Figure 1- Visual Receptors

Table 2- Difference between Visual Receptors

3.3.2 Visual Pigments

• Both rods and cones contain light-sensitive pigments that decompose when they absorb light energy.
• The light-sensitive biochemical in rods is called rhodopsin, or visual purple.
• In the presence of light, rhodopsin molecules are broken down into a colorless protein called opsin and a yellowish substance called retinal (retinene) that is synthesized from vitamin A.
• As in rods, the light-sensitive pigments in cones are composed of retinal and protein.
• In cones, however, three different opsin proteins, different from that found in rods, combine with retinal to form the three cone pigments.
• The three types of cones each contain one of these three visual pigments.
• One type of cone pigment (erythrolabe) is most sensitive to red light waves, another (chlorolabe) to green light waves, and a third (cyanolabe) to blue light waves.