The adult eyeball measures about 2.5 cm (1 in.) in diameter. Of its total surface area, only the anterior one-sixth is exposed; the remainder is recessed and protected by the orbit, into which it ﬁts.
Anatomically, the wall of the eyeball consists of three layers: (1) ﬁbrous tunic, (2) vascular tunic, and (3) retina.
The ﬁbrous tunic is the superﬁcial layer of the eyeball and consists of the anterior cornea and posterior sclera.
The cornea is a transparent coat that covers the colored iris. Because it is curved, the cornea helps focus light onto the retina. Its outer surface consists of non-keratinized stratiﬁed squamous epithelium. The middle coat of the cornea consists of collagen ﬁbers and ﬁbroblasts, and the inner surface is simple squamous epithelium. Since the central part of the cornea receives oxygen from the outside air, contact lenses that are worn for long periods of time must be permeable to permit oxygen to pass through them. The sclera (hard), the “white” of the eye, is a layer of dense connective tissue made up mostly of collagen ﬁbers and ﬁbroblasts. The sclera covers the entire eyeball except the cornea; it gives shape to the eyeball, makes it more rigid, protects its inner parts, and serves as a site of attachment for the extrinsic eye muscles. At the junction of the sclera and cornea is an opening known as the scleral venous sinus (canal of Schlemm). A ﬂuid called aqueous humor drains into this sinus.
The vascular tunic or uvea is the middle layer of the eyeball. It is composed of three parts: choroid, ciliary body, and iris. The highly vascularized choroid, which is the posterior portion of the vascular tunic, lines most of the internal surface of the sclera. Its numerous blood vessels provide nutrients to the posterior surface of the retina. The choroid also contains melanocytes that produce the pigment melanin, which causes this layer to appear dark brown in color. Melanin in the choroid absorbs stray light rays, which prevents reﬂection and scattering of light within the eyeball. As a result, the image cast on the retina by the cornea and lens remains sharp and clear. Albinos lack melanin in all parts of the body, including the eye. They often need to wear sunglasses, even indoors, because even moderately bright light is perceived as bright glare due to light scattering.
In the anterior portion of the vascular tunic, the choroid becomes the ciliary body. It extends from the ora serrata , the jagged anterior margin of the retina, to a point just posterior to the junction of the sclera and cornea. Like the choroid, the ciliary body appears dark brown in color because it contains melanin-producing melanocytes. In addition, the ciliary body consists of ciliary processes and ciliary muscle. The ciliary processes are protrusions or folds on the internal surface of the ciliary body. They contain blood capillaries that secrete aqueous humor. Extending from the ciliary process are zonular ﬁbers (suspensory ligaments) that attach to the lens. The ciliary muscle is a circular band of smooth muscle. Contraction or relaxation of the ciliary muscle changes the tightness of the zonular ﬁbers, which alters the shape of the lens, adapting it for near or far vision.
The iris (rainbow), the colored portion of the eyeball, is shaped like a ﬂattened donut. It is suspended between the cornea and the lens and is attached at its outer margin to the ciliary processes. It consists of melanocytes and circular and radial smooth muscle ﬁbers. The amount of melanin in the iris determines the eye color. The eyes appear brown to black when the iris contains a large amount of melanin, blue when its melanin concentration is very low and green when its melanin concentration is moderate.
A principal function of the iris is to regulate the amount of light entering the eyeball through the pupil, the hole in the center of the iris. The pupil appears black because, as you look through the lens, you see the heavily pigmented back of the eye (choroid and retina). However, if bright light is directed into the pupil, the reﬂected light is red because of the blood vessels on the surface of the retina. It is for this reason that a person’s eyes appear red in a photograph (“red eye”) when the ﬂash is directed into the pupil. Autonomic reﬂexes regulate pupil diameter in response to light levels. When bright light stimulates the eye, parasympathetic ﬁbers of the oculomotor (III) nerve stimulate the circular muscles (sphincter pupillae) of the iris to contract, causing a decrease in the size of the pupil (constriction). In dim light, sympathetic neurons stimulate the radial muscles (dilator pupillae) of the iris to contract, causing an increase in the pupil’s size (dilation).
The third and inner layer of the eyeball, the retina, lines the posterior three-quarters of the eyeball and is the beginning of the visual pathway. The anatomy of this layer can be viewed with an ophthalmoscope (of-THAL-mo¯-sko¯p; ophthalmos- eye; -skopeo to examine), an instrument that shines light into the eye and allows an observer to peer through the pupil, providing a magniﬁed image of the retina and its blood vessels as well as the optic (II) nerve. The surface of the retina is the only place in the body where blood vessels can be viewed directly and examined for pathological changes, such as those that occur with hypertension, diabetes mellitus, cataracts, and age-related macular disease. Several landmarks are visible through an ophthalmoscope.
The optic disc is the site where the optic (II) nerve exits the eyeball. Bundled together with the optic nerve are the central retinal artery, a branch of the ophthalmic artery, and the central retinal vein. Branches of the central retinal artery fan out to nourish the anterior surface of the retina; the central retinal vein drains blood from the retina through the optic disc. Also visible are the macula lutea and fovea centralis, which are described shortly. The retina consists of a pigmented layer and a neural layer. The pigmented layer is a sheet of melanin-containing epithelial cells located between the choroid and the neural part of the retina. The melanin in the pigmented layer of the retina, like in the choroid, also helps to absorb stray light rays. The neural (sensory) layer of the retina is a multilayered outgrowth of the brain that processes visual data extensively before sending nerve impulses into axons that form the optic nerve. Three distinct layers of retinal neurons the photoreceptor layer, the bipolar cell layer, and the ganglion cell layer are separated by two zones, the outer and inner synaptic layers, where synaptic contacts are made.
Note that light passes through the ganglion and bipolar cell layers and both synaptic layers before it reaches the photoreceptor layer. Two other types of cells present in the bipolar cell layer of the retina are called horizontal cells and amacrine cells. These cells form laterally directed neural circuits that modify the signals being transmitted along the pathway from photoreceptors to bipolar cells to ganglion cells.
Photoreceptors are specialized cells that begin the process by which light rays are ultimately converted to nerve impulses.
There are two types of photoreceptors: rods and cones. Each retina has about 6 million cones and 120 million rods. Rods allow us to see in dim light, such as moonlight. Because rods do not provide color vision, in dim light we can see only black, white, and all shades of gray in between. Brighter lights stimulate cones, which produce color vision. Three types of cones are present in the retina: (1) blue cones, which are sensitive to blue light, (2) green cones, which are sensitive to green light, and (3) red cones, which are sensitive to red light. Color vision results from the stimulation of various combinations of these three types of cones. Most of our experiences are mediated by the cone system, the loss of which produces legal blindness. A person who loses rod vision mainly has difﬁculty seeing in dim light and thus should not drive at night. From photoreceptors, information ﬂows through the outer synaptic layer to bipolar cells and then from bipolar cells through the inner synaptic layer to ganglion cells. The axons of ganglion cells extend posteriorly to the optic disc and exit the eyeball as the optic (II) nerve. The optic disc is also called the blind spot. Because it contains no rods or cones, we cannot see an image that strikes the blind spot.
Normally, you are not aware of having a blind spot, but you can easily demonstrate its presence. Hold this page about 20 in. from your face with the cross shown below directly in front of your right eye. You should be able to see the cross and the square when you close your left eye. Now, keeping the left eye closed, slowly bring the page closer to your face while keeping the right eye on the cross. At a certain distance the square will disappear from your ﬁeld of vision because its image falls on the blind spot.
The macula lutea is in the exact center of the posterior portion of the retina, at the visual axis of the eye. The fovea centralis, a small depression in the center of the macula lutea, contains only cones. In addition, the layers of bipolar and ganglion cells, which scatter light to some extent, do not cover the cones here; these layers are displaced to the periphery of the fovea centralis. As a result, the fovea centralis is the area of highest visual acuity or resolution (sharpness of vision). A main reason that you move your head and eyes while looking at something is to place images of interest on your fovea centralis—as you do to read the words in this sentence! Rods are absent from the fovea centralis and are more plentiful toward the periphery of the retina. Because rod vision is more sensitive than cone vision, you can see a faint object (such as a dim star) better if you gaze slightly to one side rather than looking directly at it.
Behind the pupil and iris, within the cavity of the eyeball, is the lens. Within the cells of the lens, proteins called crystallins, arranged like the layers of an onion, make up the refractive media of the lens, which normally is perfectly transparent and lacks blood vessels. It is enclosed by a clear connective tissue capsule and held in position by encircling zonular ﬁbers, which attach to the ciliary processes. The lens helps focus images on the retina to facilitate clear vision.
Interior of the Eyeball The lens divides the interior of the eyeball into two cavities: the anterior cavity and vitreous chamber. The anterior cavity the space anterior to the lens consists of two chambers. The anterior chamber lies between the cornea and the iris. The posterior chamber lies behind the iris and in front of the zonular ﬁbers and lens. Both chambers of the anterior cavity are ﬁlled with aqueous humor, a transparent watery ﬂuid that nourishes the lens and cornea. Aqueous humor continually ﬁlters out of blood capillaries in the ciliary processes of the ciliary body and enters the posterior chamber. It then ﬂows forward between the iris and the lens, through the pupil, and into the anterior chamber. From the anterior chamber, aqueous humor drains into the scleral venous sinus (canal of Schlemm) and then into the blood. Normally, aqueous humor is completely replaced about every 90 minutes. The larger posterior cavity of the eyeball is the vitreous chamber, which lies between the lens and the retina. Within the vitreous chamber is the vitreous body, a transparent jellylike substance that holds the retina ﬂush against the choroid, giving the retina an even surface for the reception of clear images. It occupies about four-ﬁfths of the eyeball.
Unlike the aqueous humor, the vitreous body does not undergo constant replacement. It is formed during embryonic life and consists of mostly water plus collagen ﬁbers and hyaluronic acid. The vitreous body also contains phagocytic cells that remove debris, keeping this part of the eye clear for unobstructed vision. Occasionally, collections of debris may cast a shadow on the retina and create the appearance of specks that dart in and out of the ﬁeld of vision. These vitreal ﬂoaters, which are more common in older individuals, are usually harmless and do not require treatment. The hyaloid canal is a narrow channel that is inconspicuous in adults and runs through the vitreous body from the optic disc to the posterior aspect of the lens. In the fetus, it is occupied by the hyaloid artery. The pressure in the eye, called intraocular pressure, is produced mainly by the aqueous humor and partly by the vitreous body; normally it is about 16 mmHg (millimeters of mercury). The intraocular pressure maintains the shape of the eyeball and prevents it from collapsing. Puncture wounds to the eyeball may cause the loss of aqueous humor and the vitreous body. This in turn causes a decrease in intra ocular pressure, a detached retina, and in some cases blindness.
SECTIONAL VIEW OF HUMAN EYE BALL
PUPIL DYNAMICS TO REGULATE THE LIGHT INTENSITY
OPHTHALMOSCOPE VIEW OF EYE
ARRANGEMENT OF AQUEOUS & VITREOUS CHAMBER
MICROSCOPIC VIEW OF RETINA
MICROSCOPIC VIEW OF OPTIC NERVE REGION
HISTOLOGY OF PORTION OF RETINA