Structure of the

The eye is a hollow, spherical structure about 2.5 centimeters in diameter. Its wall has three distinct layers— an outer fibrous tunic, a middle vascular tunic, and an inner nervous tunic. The spaces within the eye are filled with fluids that support its wall and internal structures and help maintain its shape. Figure 12.26 shows the major parts of the eye.

The Outer Tunic The anterior sixth of the outer tunic bulges forward as the transparent cornea (kor'ne-ah), which is the window of the eye and helps focus

Vitreous humor

Suspensory ligaments Iris

Cornea

Vitreous humor

Suspensory ligaments Iris

Cornea

Anterior -cavity

Ciliary body

Posterior Cavity

Medial rectus

Sclera Optic disk

Optic nerve

Fovea centralis

Posterior cavity Retina Choroid coat

Lateral rectus

Figure 12.26

Transverse section of the left eye (superior view).

Pupil Lens Aqueous humor

Anterior -cavity

Ciliary body

Lateral rectus

Medial rectus

Sclera Optic disk

Optic nerve

Fovea centralis

Posterior cavity Retina Choroid coat

Figure 12.26

Transverse section of the left eye (superior view).

entering light rays. It is largely composed of connective tissue with a thin surface layer of epithelium. The cornea is transparent because it contains few cells and no blood vessels. The cells and collagenous fibers form unusually regular patterns.

In contrast, the cornea is well supplied with nerve fibers that enter its margin and radiate toward its center. These fibers are associated with many pain receptors that have very low thresholds. Cold receptors are also abundant in the cornea, but heat and touch receptors are not.

Along its circumference, the cornea is continuous with the sclera (skle'rah), the white portion of the eye. The sclera makes up the posterior five-sixths of the outer tunic and is opaque due to many large, haphazardly arranged collagenous and elastic fibers. The sclera protects the eye and is an attachment for the extrinsic muscles.

In the back of the eye, the optic (op'tik) nerve and blood vessels pierce the sclera. The dura mater that encloses these structures is continuous with the sclera.

The Middle Tunic The middle, or vascular, tunic of the eyeball (uveal layer) includes the choroid coat, the ciliary body, and the iris. The choroid coat, in the posterior five-sixths of the globe of the eye, loosely joins the sclera. Blood vessels pervade the choroid coat and nourish surrounding tissues. The choroid coat also contains numerous pigment-producing melanocytes that give it a brownish black appearance. The melanin of these cells absorbs excess light and helps keep the inside of the eye dark.

In 1905, doctors transplanted the cornea of an eleven-year-old boy who lost his eye in an accident into a man whose cornea had been destroyed by a splash of a caustic chemical, marking one of the first successful human organ transplants. Today, corneal transplants are commonly used to treat corneal disease, the most common cause of blindness worldwide. In this procedure, called a penetrating keratoplasty, a piece of donor cornea replaces the central two-thirds of the defective cornea. These transplants are highly successful because the cornea lacks blood vessels, and therefore the immune system does not have direct access to the new, "foreign" tissue. Unfortunately, as is the case for many trans-plantable body parts, donor tissue is in short supply.

Cornea

Conjunctiva Iris

Posterior chamber

Suspensory ligaments

Vitreous humor

Figure 12.27

Anterior portion of the eye.

Anterior chamber

Ciliary process — Ciliary muscles —

Ciliary body

Cornea

Anterior chamber

Ciliary process — Ciliary muscles —

Ciliary body

Sclera

Posterior chamber

Suspensory ligaments

Vitreous humor

Choroid Coat Ligament Eye

Sclera

The ciliary body, which is the thickest part of the middle tunic, extends forward from the choroid coat and forms an internal ring around the front of the eye. Within the ciliary body are many radiating folds called ciliary processes and two distinct groups of muscle fibers that constitute the ciliary muscles. Figure 12.27 shows these structures.

Many strong but delicate fibers, called suspensory ligaments (zonular fibers), extend inward from the ciliary processes and hold the transparent lens in position. The distal ends of these fibers are attached along the margin of a thin capsule that surrounds the lens. The body of the lens, which lacks blood vessels, lies directly behind the iris and pupil and is composed of specialized epithelial cells.

The cells of the lens originate from a single layer of epithelium beneath the anterior portion of the lens capsule. The cells divide, and the new cells on the surface of the lens capsule differentiate into columnar cells called lens fibers, which constitute the substance of the lens. Lens fiber production continues slowly throughout life, thickening the lens from front to back. Simultaneously, the deeper lens fibers are compressed toward the center of the structure (fig. 12.28).

The lens capsule is a clear, membranelike structure largely composed of intercellular material. It is quite elastic, a quality that keeps it under constant tension. As a result, the lens can assume a globular shape. However, the suspensory ligaments attached to the margin of the capsule are also under tension, and they pull outward, flattening the capsule and the lens (fig. 12.29).

If the tension on the suspensory ligaments relaxes, the elastic capsule rebounds, and the lens surface becomes more convex. This change occurs in the lens when the eye focuses to view a close object and is called accommodation (ah-kom"o-da'shun).

Eye Lens Fibers

Figure

A scanning electron micrograph of the long, flattened lens fibers (2,650x). Note the fingerlike junctions where one fiber joins another.

Figure

A scanning electron micrograph of the long, flattened lens fibers (2,650x). Note the fingerlike junctions where one fiber joins another.

The ciliary muscles relax the suspensory ligaments during accommodation. One set of these muscle fibers forms a circular sphincterlike structure around the ciliary processes. The fibers of the other set extend back from fixed points in the sclera to the choroid coat. When the circular muscle fibers contract, the diameter of the ring formed by the ciliary processes decreases; when the

Ciliary processes of ciliary body

Lens

Retina

Ciliary processes of ciliary body

Lens

Retina

Ciliary Body Anatomy

Suspensory' — \ Choroid coat ligaments Sclera

Figure 12.29

The lens and ciliary body viewed from behind.

Suspensory' — \ Choroid coat ligaments Sclera

Figure 12.29

The lens and ciliary body viewed from behind.

other fibers contract, the choroid coat is pulled forward, and the ciliary body shortens. Both of these actions relax the suspensory ligaments, thickening the lens. In this thickened state, the lens is focused for viewing closer objects than before (fig. 12.30).

To focus on a distant object, the ciliary muscles relax, increasing tension on the suspensory ligaments. The lens thins again.

Ciliary Muscle Lens Relaxed Contracted

Ciliary muscle fibers relaxed

Suspensory ligaments taut

Lens thin

Ciliary muscle fibers contracted

Suspensory ligaments relaxed

Lens thick

Figure

Figure

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Responses

  • Lisa
    Why is the choroid coat brownishblack in appearance?
    6 years ago
  • SOFIA
    When focusing on a distant object the choroid, ciliary muscles and suspensory ligaments?
    6 years ago
  • alfrida
    What parts of the eye have blood vessels?
    5 years ago
  • gianfranco
    Which structure of the eye contains blood vessels and lymphatic vessels?
    5 years ago
  • kimberley
    Which component of the fibrous tunic lacks blood vessels?
    5 years ago
  • rowan
    Has pigment producing cells and blood vessels,keeps inside of eye dark?
    4 years ago
  • temshe
    Where are the main blood vessels in the eye cavity?
    4 years ago
  • FOLCARD
    What structures in the choroid contain elastic tissues?
    4 years ago
  • nuguse
    What structure of the body that does not contain blood vessels?
    4 years ago
  • Sebhat
    Which blood vessels pierce the sclera?
    4 years ago
  • Brigitte
    Which structure does pierce sclera?
    3 years ago
  • florian
    Does sclrra contain nerve and blood vrssel?
    3 years ago
  • grossman
    What part of the fibrous tunic that lacks blood vessels?
    2 years ago
  • KARITA
    Which layer/structure of the eye contains blood vessels and lymphatic vessels?
    1 month ago

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