The eye is supported by the following accessory organs:
- The eyebrows shade the eyes and help keep perspiration that accumulates on the forehead from running into the eyes.
- The eyelids (palpebrae) lubricate, protect, and shade the eyeballs. Contraction of the levator palpebrae superioris muscle raises the upper eyelid. Each eyelid is supported internally by a layer of connective tissue, the tarsal plate. Tarsal (Meibomian) glands embedded in the tarsal plate produce secretions that prevent the upper and lower eyelids from sticking together. The inner lining of the eyelid, the conjunctiva, is a mucous membrane that produces secretions that lubricate the eyeball. The conjunctiva continues beyond the eyelid, folding back to cover the white of the eye.
- The eyelashes, on the borders of the eyelids, help protect the eyeball. Nerve endings at the base of the hairs initiate a reflex action that closes the eyelids when the eyelashes are disturbed.
- The lacrimal apparatus produces and drains tears. Tears (lacrimal fluid) are produced by the lacrimal glands, which lie above each eye (toward the lateral edge). In each eye, tears flow across the eyeball and enter two openings (lacrimal puncta) into lacrimal canals that lead to the lacrimal sac. From here, the tears drain through the nasolacrimal duct into the nasal cavity. Tears contain antibodies and lysozyme (a bacteria‐destroying enzyme).
- Six extrinsic eye muscles provide fine motor control for the eyeballs. These are the lateral, medial, superior, and inferior rectus muscles and the inferior and superior oblique muscles.
The eyeball is a hollow sphere whose wall consists of three tunics (layers), shown in Figure 1.
The three tunics of the eye are described below:
Fibrous tunic: The outer fibrous tunic consists of avascular connective tissue called the sclera. The forward 1/6 portion of this tunic is the cornea, a transparent layer of collagen fibers that forms a window for entering light. The remainder of the fibrous tunic is the sclera. Consisting of tough connective tissue, the sclera maintains the shape of the eyeball and provides for the attachment of the eye muscles. The visible forward portion of the sclera is the white of the eye.
Vascular tunic: The middle vascular tunic (uvea) consists of three highly vascularized (as the name implies), pigmented parts (the iris, the ciliary body, and the choroid):
- The iris is the colored portion of the eye that opens and closes to control the size of its circular opening, the pupil. The size of the pupil regulates the amount of light entering the eye and helps bring objects into focus.
- The ciliary body lies between the iris and the choroids (the remainder of the vascular tunic). The ciliary processes that extend from the ciliary body secrete aqueous humor, the fluid that fills the forward chamber of the eye. The suspensory ligament between the ciliary processes and the lens holds the lens in place, while ciliary muscles (in the ciliary body) that pull on the suspensory ligament control the shape of the lens to focus images.
- The choroid connects with the ciliary body at a jagged boundary and forms the remaining portion (5/6) of the vascular tunic. The choroid is dark brown, absorbing light and reducing reflection within the chamber of the eyeball that would otherwise blur images. The highly vascularized choroid provides nutrients to surrounding tissues, including the avascularized fibrous tunic.
Nervous tunic: The inner nervous tunic is the retina. The retina consists of an outer pigmented epithelium covered by nervous tissue (the neural layer) on the inside. The dark color of the pigmented epithelium absorbs light (as with the choroid) and stores vitamin A used by photoreceptor cells in the neural layer. There are two kinds of photoreceptors in the retina:
- Cones are photoreceptor cells that respond to bright light and color. They transmit sharp images. The concentration of cones is low at the periphery of the retina and increases as the cones approach the macula lutea, an oval region in the center of the rear portion of the retina. The center of the macula lutea, the fovea centralis, contains only cones; other retinal cells are absent, exposing the cones directly to incoming light. The high concentration of cones and direct exposure to light make the fovea centralis the site on the retina that provides the highest visual acuity. As a result, images that are viewed directly are focused upon the fovea centralis.
- Rods are photoreceptor cells that are more sensitive to light and more numerous than cones. As a result, rods provide vision in dim light. They are also more capable of detecting movement; however, rods cannot detect color, and dimly lit objects appear gray. Because the concentration of rods increases in areas farther away from the macula lutea, detecting a moving or dimly lit object can be more effectively achieved by looking slightly away from the object.
Figure 1. Details of the eye and the retina.
Within the nervous tunic, photoreceptor cells (rods and cones) form synapses with other nerve cells (refer to Figure 1). When stimulated by light, rods and cones pass graded potentials to bipolar cells, which in turn pass graded potentials to the ganglion cells. The graded potentials may be modified by horizontal cells and amacrine cells that link adjacent photoreceptors or ganglion cells, respectively. Action potentials are ultimately generated by ganglion cells. The axons of all the ganglion cells gather at the optic disc and exit the nervous tunic through the optic disc as the optic nerve. The optic disc is a blind spot because photoreceptors are absent here.
The lens of the eye consists of tightly packed cells arranged in successive layers (as in an onion) and filled with transparent proteins called crystallins. The lens divides the interior of the eyeball into two cavities:
- The anterior cavity, the area in front of the lens, is subdivided by the iris and ciliary body into the anterior chamber and the posterior chamber. Capillaries in the ciliary body produce a clear fluid, the aqueous humor, which flows into the posterior chamber, through the pupil, and into the anterior chamber. The aqueous humor then drains into veins through a channel (scleral venous sinus, or canal of Schlemm) that encircles the eye where the cornea and sclera join. The aqueous humor, which is continuously replaced, provides pressure to maintain the shape of the forward portion of the eye and supplies O 2 and nutrients to the avascular lens and cornea.
- The posterior cavity, the area behind the lens, is filled with a clear gel, the vitreous humor. The vitreous humor, which is produced during embryonic development and is not replaced, holds the lens and retina in position and maintains the shape of the eye.
The process of sight involves converting light energy to chemical energy. Features of the process follow:
- The outer segments of rods and cones contain numerous folds that increase the surface area exposed to light.
- Photopigments (visual pigments) in the outer segments respond to light by changing their chemical structure. Each visual photopigment consists of two parts: retinal (a vitamin A derivative) and opsin (a glycoprotein). There are four different kinds of photopigments because the opsin in each has a slightly different structure, enabling each photopigment to absorb a different range of light wavelengths (different colors).
- There are three kinds of cones, each possessing a different photopigment, and each sensitive to a different range of light wavelengths. A mixed stimulation of the three different cones (called red, green, and blue for their optimally absorbed wavelengths) provides for the perception of varied colors.
- There is only one kind of rod, with a single kind of photopigment, rhodopsin.
- When a photopigment absorbs light, retinal changes shape, causing it to separate from opsin. Because the product is colorless, the process is called bleaching.
- Unlike most other neurons, photoreceptors continually secrete a neurotransmitter when in the resting (unstimulated), polarized condition. When a photoreceptor is stimulated, the freed opsin becomes chemically active and initiates a series of chemical reactions that close the Na + channels in the plasma membrane. Because the Na +/K + pump continues to pump Na + out of the cell, the plasma membrane becomes hyperpolarized. Hyperpolarization stops the normal secretion of the neurotransmitter, which in turn stimulates a graded potential in the bipolar cells.
- Photopigments are regenerated when opsin is enzymatically reattached to retinal. In bright light, the very light‐sensitive rhodopsin in rods cannot be regenerated as fast as it is broken down, so most of the rhodopsin remains inactive. As a result, the less light‐sensitive pigments in cones are active in normal daylight. When eyes move from bright conditions to dark conditions, the photopigments in cones are too insensitive to detect light, and the rhodopsin in rods is still bleached from its exposure to bright light. Vision returns as rhodopsin is regenerated, a process called dark adaptation. When eyes move from dark to bright conditions, very light‐sensitive rods are suddenly overwhelmed with stimulation, producing the sensation of glare. Light adaptation occurs as the rhodopsin in rods is completely bleached and the less light‐sensitive cones resume activity.
When light reflected from an object enters the eye, the following processes occur:
Light refraction: When light rays pass from one substance to another substance of different density, the rays bend, or refract. The amount of bending depends on the angle of incidence of the light ray and the degree to which the densities of the two substances differ. When distant objects are sighted, the normal curvature of the lens appropriately compensates for the refraction due to the differences in densities among the aqueous humor, the lens, and the vitreous humor.
Lens accommodation: Light rays from near objects enter the eyeball at more divergent angles than rays from distant objects. Thus, when near objects are sighted, muscles pull on the lens to increase its curvature so that the more divergent rays of the close object are properly refracted upon the retina.
Pupil constriction: One function of the pupil is to regulate the amount of light that enters the posterior cavity so that the retina receives the appropriate amount of stimulation. In addition, when near objects are sighted, the pupil constricts (accommodation pupillary reflex) to block the most divergent light rays that cannot be brought into focus by the accommodation of the lens. Reading under low levels of light may be difficult because the pupils are dilated to allow all available light to enter rather than constricted to improve focusing. Increasing the amount of light improves the ability to read because the surplus light permits the pupils to constrict and the lens to focus a more narrow beam of light rays.
Eyeball convergence: Both eyes point in the same direction when viewing a distant object. When near objects are sighted, the eyes must be directed medially to simultaneously view the object, a process called convergence.
Nerve impulses generated by visual stimuli travel along the axons of ganglion cells within the two optic nerves. Before entering the brain, axons representing the medial portions of the visual fields of each eye cross over at the optic chiasma. After the crossover, the axons, now forming the optic tract, enter the thalamus. Processed visual stimuli are then carried to visual areas of the occipital lobes of both cerebral hemispheres by nerve pathways called the optic radiations. Because of the partial crossover at the optic chiasma, each cerebral hemisphere receives the lateral portion of the visual field of the eye on the same side of the body as the cerebral hemisphere, but the medial portion of the visual field of the eye is interpreted on the opposite side of the body. In addition, because of the action of the lens, the image that forms on the retina and that is sent to the brain is inverted and reversed right to left. The brain, however, interprets all of this seemingly disparate visual information into a coherent perception of the real world.