Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
The Organ of Vision. The Visual Analyzer

The Visual sensory system, along with the auditory system, plays a vital role in human cognitive activity. Through the visual analyzer, humans receive up to 90% of information about the surrounding world. The activities of the visual analyzer are associated with the following Functions: photosensitivity, determination of object shapes, their sizes, the distance of objects from the eye, motion perception, Color Vision, and binocular vision.

Structure AND FUNCTIONS of The Organ of vision. The Organ of vision consists of the Eyeball (eye) and the Accessory structures of the eye located within the Orbit.

The eyeball is spherical in shape. It consists of three tunics and a core (Fig. 107). The outer tunic is fibrous, the middle is vascular, and the inner is photosensitive and reticular (the retina). The core of the eyeball includes the lens, the vitreous body, and a fluid medium known as aqueous humor.

The fibrous tunic is thick and dense, divided into two sections: anterior and posterior. The anterior section occupies 1/₅ of the eyeball's surface. It is formed by the transparent cornea, which bulges anteriorly. The cornea lacks Blood Vessels and possesses high light-refracting properties. The posterior section of the fibrous tunic is the sclera, which resembles the color of a boiled chicken egg white. The sclera is formed by dense Fibrous Connective Tissue. The vascular tunic (uvea) lies beneath the sclera and consists of three structurally and functionally distinct parts: the choroid proper, the ciliary body, and the iris. The choroid proper occupies the large posterior part of the eye. It is thin, rich in blood vessels, and contains pigment Cells that give it a dark brown color.

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Fig. 107. The eyeball. Horizontal section (diagram): The lens during relaxation (A) and contraction (B) of the ciliary Muscle: 1 — cornea; 2 — anterior chamber of the eye; 3 — lens; 4 — iris; 5 — posterior chamber of the eye; 6 — conjunctiva; 7 — lateral rectus muscle; 8 — sclera; 9 — choroid; 10 — retina; 11 — fovea centralis; 12 — Optic nerve; 13 — optic disc (blind spot); 14 — outer axis of the eye; 15 — medial rectus muscle; 16 — transverse axis of the eye; 17 — ciliary body; 18 — zonule of Zinn; 19 — visual axis of the eye

The ciliary body is located anterior to the choroid proper and has the appearance of a thickened ridge. From the anterior edge of the ciliary body, projections extend toward the lens—ciliary processes and delicate fibers (the ciliary zonule), which attach to the lens capsule along its equator. The major part of the ciliary body consists of the ciliary muscle. When this muscle contracts, it alters the tension of the zonular fibers, thereby regulating the curvature of the lens and changing its refractive power. The iris is situated between the cornea anteriorly and the lens posteriorly. It resembles a frontally positioned disc with an aperture (the pupil) in the center. At its outer edge, the iris merges into the ciliary body. The inner, free margin of the iris bounds the pupil aperture. The connective tissue stroma of the iris contains blood vessels, smooth muscle cells, and pigment cells. Eye color—hazel, brown, black (with a high pigment concentration), blue, or greenish (when pigment is sparse)—depends on the amount and depth of the pigment. Bundles of smooth muscle cells run in two directions, forming the dilator pupillae muscle (which widens the pupil) and the sphincter pupillae muscle (which narrows the pupil). These Muscles regulate The entry of light into the eye.

The retina adheres internally to the vascular tunic. Two parts are distinguished in the retina: the posterior optical part and the anterior ciliary and iridial parts. The posterior optical part contains photosensitive cells known as photoreceptors. The anterior part of the retina (the blind part) lies adjacent to the ciliary body and the iris; it contains no photosensitive cells. The optical part of the retina has a complex structure, consisting of two layers: an inner photosensitive layer and an outer pigmented layer. The Cells of the pigmented layer participate in absorbing light that enters THE EYE AND passes through the photosensitive retinal layer. The inner layer of the retina represents three layers of Nerve Cells: the outer photoreceptor layer (adjacent to the pigmented layer), the middle associative layer, and the inner ganglion layer. The photoreceptor layer of the retina consists of neurosensory rods and cone-shaped cells, whose outer segments (dendrites) have the shape of rods or cones. The disc-like structures of rod and cone neurocytes contain photopigment molecules: black-and-white sensitive pigments in rods, and red-, green-, and blue-sensitive pigments in cones. The number of cones in the human retina reaches 6–7 million, while the number of rods is 20 times greater. Rods perceive information about the shape and illumination of objects, whereas cones perceive color information.

The central processes (axons) of the neurosensory cells (rods and cones) transmit visual impulses to the bipolar cells of the second Cell layer of the retina, which make contact with the ganglion neurocytes of the third (ganglion) retinal layer.

The ganglion layer consists of large neurocytes whose axons form the optic nerve.

In the posterior part of the retina, two areas stand out: the blind spot and the macula (yellow spot). The blind spot is the site where the optic nerve exits the eyeball; here, the retina contains no photosensitive elements. The macula is located in the region of the posterior pole of the eye and is the most light-sensitive area of the retina. The center of its depression is called the fovea centralis. The line connecting the center of the anterior pole of the eye to the fovea centralis is called the optical axis of the eye. For optimal vision, the eye uses extraocular muscles to align itself so that the observed object and the fovea centralis lie on the same axis.

As already noted, the core of the eyeball includes the lens, the vitreous body, and the aqueous humor.

The lens is a transparent biconvex lens approximately 9 mm in diameter, located behind the iris. Between the lens posteriorly and the iris anteriorly lies the posterior chamber of the eye, which contains a transparent fluid—the aqueous humor. Behind the lens is the vitreous body. The substance of the lens is colorless, transparent, and dense; it lacks blood Vessels and nerves. The lens is enclosed in a transparent capsule connected to the ciliary body by the ciliary zonule. When the ciliary muscle contracts or relaxes, the tension of the zonular fibers decreases or increases, leading to A change in the curvature of the lens and its refractive power.

The vitreous body fills the entire cavity of the eyeball between the retina posteriorly and the lens anteriorly. It consists of a transparent jelly-like substance and contains no blood vessels.

Aqueous humor is secreted by the Blood vessels of the ciliary processes. It fills the posterior and anterior Chambers of the eye, which communicate through the aperture in the iris known as the pupil. Aqueous humor flows from the posterior chamber into the anterior chamber, and from the anterior chamber into the Veins at the junction of the cornea and the sclera.