The eyes, housed in the bony orbits, are the photosensitive organs responsible for vision. Each eye is composed of three tunics: the tunica fibrosa (outer layer), tunica vasculosa (middle layer), and retina.It receives light through the cornea. The light is focused by the lens on the retina, which contains specialized cells that encode the various patterns of the image for transmission to the brain via the optic nerve.
The eye possesses intrinsic muscles that adjust the aperture of the iris and alter the lens diameter, permitting accommodation for close vision.It possesses extrinsic muscles, attached to the external aspect of the orb (eyeball), which move the eyes in a coordinated manner to perceive the desired visual fields.
The orb is continually moistened on its anterior surface with lacrimal fluid (tears) secreted by the lacrimal gland. The eyeball is covered by the upper and lower eyelids, which protect its anterior surface.
Tunica fibrosa
1. The outermost tunica fibrosa is composed of the sclera and the cornea. The sclera is an opaque, relatively avascular fibrous connective tissue layer that covers the posterior five sixths of the eyeball, the sclera receives insertions of the extrinsic ocular muscles.
2. The cornea is the transparent, highly innervated avascular anterior one-sixth of the tunica. It joins the sclera in a region called the limbus, which is highly vascularized and is composed of five layers.
a. Corneal epithelium
(1) The corneal epithelium lines the anterior aspect of the cornea and is continuous with the conjunctiva (a mucous membrane covering the anterior sclera and lining the internal surface of the eyelids).
(2) It is a stratified squamous nonkeratinized epithelium.
(3) It possesses microvilli in its superficial layer; the microvilli trap moisture, protecting the cornea from dehydration.
b. Bowman membrane
is a homogeneous noncellular layer that provides form, stability, and strength to the cornea.c. Corneal stroma
(1) The corneal stroma is the thickest corneal layer. It is composed of many layers of type I collagen fibers lying parallel to each other within each row and at oblique angles to each other in adjoining layers. The collagen fibers are embedded in a ground substance composed mostly of chondroitin sulfate and keratan sulfate. Thin elastic fibers and fibroblasts are also present in the corneal stroma.
(2) The stroma has channels in the region of the limbus that are lined by endothelium, forming the canal of Schlemm. This canal drains fluid (aqueous humor) from the anterior chamber of the eye into the venous system.
d. Descemet membrane
is a thick (5–10 _m) basal lamina separating the stroma from the endothelium lining the cornea.
e. Corneal endothelium
(1) The corneal endothelium lines the posterior aspect of the cornea.
(2) It is a simple squamous epithelium with cells that exhibit numerous pinocytic vesicles.
(3) It resorbs fluid from the stroma, thus contributing to the transparency of the cornea, contributory to light refraction.
C. Tunica vasculosa
(uvea, the middle tunic) is composed of three parts:
1. Choroid
a. The choroid is the highly vascular, pigmented layer of the eye on the posterior wall of the orb; its loose connective tissue contains many melanocytes.
b. It is loosely attached to the tunica fibrosa.
c. It possesses a deep choriocapillary layer and Bruch membrane (basement membrane), which extends from the optic disk to the ora serrata.
2. Ciliary body
a. The ciliary body is the wedge-shaped anterior expansion of the choroid.
b. It completely encircles the lens and separates the ora serrata from the iris.
c. It is lined on its inner surface by two layers of cells: an outer pigmented columnar epithelium rich in melanin and an inner non pigmented simple columnar epithelium.
Ciliary processes
(a) are radially arranged extensions (about 70) of the ciliary body.
(b) have a connective tissue core containing many fenestrated capillaries.
(c) are covered by two epithelial layers. The unpigmented inner layer transports components from the plasma filtrate in the posterior chamber and thus forms the aqueous humor, which flows to the anterior chamber via the pupillary aperture.
(d) possess suspensory ligaments (zonulae) that arise from the processes and insert into the capsule of the lens, serving to anchor it in place.
Ciliary muscle
Is attached to the sclera and ciliary body in such a manner that its contractions stretch the ciliary body and release tension on the suspensory ligament and lens. Ciliary muscle contraction permits the lens to become more convex, allowing the eye to focus on nearby objects (accommodation). With advancing age, the lens loses its elasticity, thereby gradually losing the ability to accommodate.3. Iris
(1) The iris is the most anterior extension of the choroid, separating the anterior and posterior chambers of the orb.
(2) It incompletely covers the anterior surface of the lens, forming an aperture called the pupil that is continually adjusted by intrinsic pupillary muscles.
(3) The iris is covered by an incomplete layer of pigmented cells and fibroblasts on its anterior surface.
(4) It has a wall composed of loose vascular connective tissue containing melanocytes and fibroblasts.
(5) The iris is covered on its deep surface by a two-layered epithelium possessing pigmented cells that block light from entering the interior of the eye except via the pupil.
Eye color is blue if only a few melanocytes are present. Increasing amounts of pigment impart darker colors to the eye.
Dilator pupillae muscle
Dilator pupillae muscle is a smooth muscle with fibers that radiate from the periphery of the iris toward the pupil. It contracts upon stimulation by sympathetic nerve fibers, dilating the pupil.
Sphincter pupillae muscle
Sphincter pupillae muscle is smooth muscle arranged in concentric rings around the pupillary orifice.It contracts upon stimulation by parasympathetic nerve fibers of the oculomotor nerve (cranial nerve III), constricting the pupil.
D. Refractive media of the eye
1. Aqueous humor
Aqueous humor is a plasm alike fluid in the anterior compartment of the eye that is formed by epithelial cells lining the ciliary processes. It is constantly secreted into the posterior chamber of the eye and then flows into the anterior chamber of the eye via the pupillary aperture; from there it enters the venous system via the canal of Schlemm .
2. The lens
Is a biconvex transparent flexible structure composed of the lens capsule, subcapsular epithelium, and lens fibers .The lens capsule is a thick basal lamina that envelops the entire lens epithelium.The subcapsular epithelium (on the anterior and lateral lens surfaces only) is composed of a single layer of cuboidal cells .Lens fibers are filled with a group of proteins called crystallins. The suspensory ligament stretches between the lens and the ciliary body, keeping tension on the lens and enabling it to focus on distant objects.
3. The vitreous body
Is a refractile gel composed mainly of water, collagen, and hyaluronic acid, this gel fills the interior of the globe posterior to the lens .
E. Retina
The retina is the innermost of the three tunicae of the eye and is responsible for photoreception .It has a shallow depression in its posterior wall that contains only cones; this avascular region, called the fovea centralis, whose central region, the macula, exhibits the greatest visual acuity . It displays 10 distinct layers.
The photoreceptor layer consists of neurons (photoreceptor cells) called rods and cones.Their dendrites interdigitate with cells of the pigmented epithelium, and their bases form synapses with cells of the bipolar layer.
Rods (sensitive to light of low intensity)
(a) Rods have outer and inner segments, a nuclear region, and a synaptic region.
(b) They may synapse with bipolar cells.
(c) They contain an incomplete cilium terminating in a basal body within the inner segment.
The outer segments of rods
(a) consist mainly of hundreds of flattened membranous disks that contain rhodopsin.
(b) eventually shed their disks, which are subsequently phagocytized by the pigment epithelial cells.
The inner segments of rods possess mitochondria, glycogen, polyribosomes, and proteins, which migrate to the outer segments to be incorporated into the membranous disks.
Photoreception by rods is initiated by the interaction of light with rhodopsin (visual purple
Cones (sensitive to light of high intensity)
(1) Cones are much less numerous than rods but produce greater visual acuity than do rods; thus, the macula is populated only by cones.
(2) They are generally similar in structure to rods and mediate photoreception in the same way, with the following exceptions:
Cones possess iodopsin in their disks. The amount of this photopigment varies in different cones, making them differentially sensitive to red, green, or blue light.
External limiting membrane
a. The external limiting membrane is not a true membrane but an area where zonulae adherents are located between the photoreceptor cells and the retinal Müller cells (glial cells).b. This membrane also contains microvilli that project from the Müller cells.
The outer nuclear layer consists primarily of the nuclei of the rods and cones.
Outer plexiform layer
a. The outer plexiform layer contains axodendritic synapses between the axons of photoreceptor cells and the dendrites of bipolar and horizontal cells.
b. It displays synaptic ribbons within the rod and cone cells at synaptic sites.
7. The inner nuclear layer contains the cell bodies of bipolar neurons, horizontal cells, amacrine cells, and the nuclei of Müller cells.
8. Inner plexiform layer
a. The inner plexiform layer contains axodendritic synapses between the axons of bipolar cells and the dendrites of ganglion cells.
b. The processes of amacrine cells are located in this layer.
9. The ganglion cell layer contains the somata of ganglion cells, which form the final link in the retina’s neural chain.
a. Structure—Ganglion cells
(1) Ganglion cells are typical neurons that project their axons to a specific region of the retina called the optic disk.
(2) These cells are midget, diffuse, and stratified ganglion cells.
b. Function—Ganglion cells. Ganglion cells are activated by hyperpolarization of rods and cones and generate an action potential, which is transmitted to horizontal and amacrine cells and carried to the visual relay system in the brain.
10. The optic nerve fiber layer consists primarily of the unmyelinated axons of ganglion cells, which form the fibers of the optic nerve. As each fiber pierces the sclera, it acquires a myelin sheath.
11. The inner limiting membrane consists of the terminations of Müller cell processes and their basement membranes.
Accessory structures of the eye
1. Conjunctiva (transparent mucous membrane)
a. The conjunctiva lines the eyelids and is reflected onto the anterior portion of the orb up to the cornea, where it becomes continuous with the corneal epithelia
b. It is a stratified columnar epithelium possessing many goblet cells.
c. It is separated by a basal lamina from an underlying lamina propria of loose connective tissue.
2. Eyelids
a. The eyelids are lined internally by conjunctiva and externally by skin that is elastic and covers a supportive framework of tarsal plates.
b. Eyelids contain highly modified sebaceous glands (meibomian glands), smaller modified sebaceous glands (glands of Zeis), and sweat glands (glands of Moll).
3. Lacrimal apparatus
a. Lacrimal gland
(1) The lacrimal gland is a compound tubuloalveolar gland with secretory units that are surrounded by an incomplete layer of myoepithelial cells.
(2) Lacrimal fluid (tears) is mostly water, and contains lysozyme, an antibacterial enzyme. Tears drain from the lacrimal gland via 6 to 12 ducts into the conjunctival fornix, from which the tears flow over the cornea and conjunctiva, keeping them moist. Tears then enter the lacrimal puncta, leading to the lacrimal canaliculi.
b. Lacrimal canaliculi
They are lined by a stratified squamous epithelium and unite to form a
common canaliculus, which empties into the lacrimal sac.
c. The lacrimal sac is lined by a pseudostratified ciliated columnar epithelium.
d. The nasolacrimal duct is the inferior continuation of the lacrimal sac and also is lined by a pseudostratified ciliated columnar epithelium. The duct empties into the floor of the nasal cavity.