The Eyes and Vision

Light from an observed object is focused by the cornea and lens onto the photoreceptive retina at the back of the eye.The focus is maintained on the retina at different distances between the object and the eyes by muscular contractions that change the thickness and degree of curvature of the lens.

The eyes transduce energy in the electromagnetic spectrum (fig. 10.25) into nerve impulses. Only a limited part of this spectrum can excite the photoreceptorselectromagnetic energy with wavelengths between 400 and 700 nanometers (1 nm = 10-9 m, or one-billionth of a meter) constitutes visible light. Light of longer

Clinical Investigation Clues

Remember that the doctor suggested that Ed have an audiol-ogy exam if he still experienced a hearing impairment after his cold got better.

What type of hearing loss could be detected by an audiology exam? What might cause such a hearing loss?

Gamma rays

X rays

Wavelength (A)

Ultraviolet

10 I

102 103

Infrared

Microwaves

Radio waves

Visible light

Hearing Loss Spectrum

Figure 10.25 The electromagnetic spectrum. Different parts of the electromagnetic spectrum (top) are shown in Angstrom units (IA = 10 The visible spectrum (bottom) constitutes only a small range of this spectrum, shown in nanometer units (I nm = 10-9 meter).

meter).

Figure 10.25 The electromagnetic spectrum. Different parts of the electromagnetic spectrum (top) are shown in Angstrom units (IA = 10 The visible spectrum (bottom) constitutes only a small range of this spectrum, shown in nanometer units (I nm = 10-9 meter).

meter).

1 Fox: Human Physiology, 1 10. Sensory Physiology Eighth Edition

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Chapter Ten

Table 10.4 Structures of the Eyeball

Tunic and Structure Location

Composition

Function

Fibrous tunic Sclera Cornea

Vascular tunic (uvea) Choroid Ciliary body

Posterior outer layer; white of the eye

Anterior surface of eyeball

Middle layer of eyeball

Middle layer in posterior portion of eyeball

Anterior portion of vascular tunic

Anterior portion of vascular tunic; continuous with ciliary body

Avascular connective tissue

Tightly bound elastic and collagen fibers

Tightly packed dense connective tissue—

transparent and convex Highly vascular pigmented tissue Vascular layer

Smooth muscle fibers and glandular epithelium

Pigment cells and smooth muscle fibers

Gives shape to the eyeball Supports and protects the eyeball Transmits and refracts light

Supplies blood; prevents reflection Supplies blood to eyeball Supports the lens through suspensory ligament and determines its thickness; secretes aqueous humor Regulates the diameter of the pupil, and hence the amount of light entering the vitreous chamber

Internal tunic

Inner layer of eyeball

Tightly packed photoreceptors, neurons, blood vessels, and connective tissue

Provides location and support for rods and cones

Retina Lens

(not part of any tunic)

Principal portion of internal tunic

Between posterior and vitreous chambers; supported by suspensory ligament of ciliary body

Photoreceptor neurons (rods and cones), bipolar neurons, and ganglion neurons Tightly arranged protein fibers; transparent

Photoreception; transmits impulses

Refracts light and focuses onto fovea centralis

Conjunctiva

Conjunctiva

Anterior Posterior Chamber Vitreous

-Anterior cavity iris

Posterior chamber

Zonular fibers of suspensory ligament

Vitreous chamber (posterior cavity)

Inferior rectus muscle

Fovea centralis Centra! artery Central vein

Optic nerve

-Anterior cavity iris

Posterior chamber

Zonular fibers of suspensory ligament

Vitreous chamber (posterior cavity)

Inferior rectus muscle

■ Figure 10.26 The internal anatomy of the eyeball. Light enters the eye from the right side of this figure and is focused on the retina.

wavelengths in the infrared regions of the spectrum is felt as heat but does not have sufficient energy to excite the receptors. Ultraviolet light, which has shorter wavelengths and more energy than visible light, is filtered out by the yellow color of the eye's lens. Honeybees—and people who have had their lenses removed—can see light in the ultraviolet range.

The structures of the eyeball are summarized in table 10.4. The outermost layer of the eye is a tough coat of connective tissue called the sclera, which can be seen externally as the white of the eyes. The tissue of the sclera is continuous with the transparent cornea. Light passes through the cornea to enter the anterior chamber of the eye. Light then passes through an opening called the pupil, which is surrounded by a pigmented muscle known as the iris. After passing through the pupil, light enters the lens (fig. 10.26).

The iris is like the diaphragm of a camera; it can increase or decrease the diameter of its aperture (the pupil) to admit more or less light. Constriction of the pupils is produced by contraction of circular muscles within the iris; dilation is produced by contraction of radial muscles. Constriction of the pupils results from parasympathetic stimulation through the occulomotor (III) nerve, whereas dilation results from sympathetic stimulation (fig. 10.27). Variations in the diameter of the pupil are similar in effect to variations in the f-stop of a camera.

Sensory Physiology

IN DIM LIGHT

Postganglionic sympathetic axon

Postganglionic sympathetic axon

From superior ■ cervical ganglion

IN NORMAL LIGHT

Muscle Radially Arranged

Radially arranged smooth muscle fibers of the iris

Circularly arranged smooth muscle fibers of the iris

Pupil

Radially arranged smooth muscle fibers of the iris

Circularly arranged smooth muscle fibers of the iris

Pupil

IN BRIGHT LIGHT

Ciliary ganglion

Ciliary ganglion

Pupillary Muscle And Zonule Fibers

Postganglionic parasympathetic axon

Postganglionic parasympathetic axon

From oculomotor nerve

■ Figure 10.27 Dilation and constriction of the pupil. In dim light, the radially arranged smooth muscle fibers are stimulated to contract by sympathetic neurons, dilating the pupil. In bright light, the circularly arranged smooth muscle fibers are stimulated to contract by parasympathetic neurons, constricting the pupil.

The posterior part of the iris contains a pigmented epithelium that gives the eye its color. The color of the eye is determined by the amount of pigment—blue eyes have the least pigment, brown eyes have more, and black eyes have the greatest amount of pigment. In the condition of albinism—a congenital absence of normal pigmentation caused by an inability to produce melanin pigment—the eyes appear pink because the absence of pigment allows blood vessels to be seen.

The lens is suspended from a muscular process called the ciliary body, which connects the sclera and encircles the lens. Zonular fibers (zon = girdle) suspend the lens from the ciliary body, forming a suspensory ligament that supports the lens. The space between the cornea and iris is the anterior chamber, and the space between the iris and the ciliary body and lens is the posterior chamber (fig. 10.28).

The anterior and posterior chambers are filled with a fluid called aqueous humor. This fluid is secreted by the ciliary body into the posterior chamber and passes through the pupil into the anterior chamber, where it provides nourishment to the avascular lens and cornea. The aqueous humor is drained from the anterior chamber into the scleral venous sinus (canal of Schlemm), which returns it to the venous blood (fig. 10.28). Inadequate drainage of aqueous humor can lead to excessive accumulation of fluid, which in turn results in increased intraocular pressure. This condition, called glaucoma, may produce serious damage to the retina and loss of vision.

The portion of the eye located behind the lens is filled with a thick, viscous substance known as the vitreous body, or vitreous humor. Light from the lens that passes through the vitreous body enters the neural layer, which contains photoreceptors, at the back of the eye. This neural layer is called the retina. Light that passes through the retina is absorbed by a darkly pigmented choroid layer underneath. While passing through the retina, some of this light stimulates photoreceptors, which in turn activate other neurons. Neurons in the retina contribute fibers that are gathered together at a region called the optic disc (fig. 10.29), where they exit the retina as the optic nerve. This region lacks photoreceptors, and is therefore known as the blind spot. The optic disc is also the site of entry and exit of blood vessels.

264 Chapter Ten

Aqueous

Aqueous

Aqueous Humor Production

Vitreous Posterior humor . chamber

Vitreous Posterior humor . chamber

■ Figure 10.28 The production and drainage of aqueous humor. Aqueous humor maintains the intraocular pressure within the anterior and posterior chambers. It is secreted into the posterior chamber, flows through the pupil into the anterior chamber, and drains from the eyeball through the canal of Schlemm.

Canal Schlemm

■ Figure 10.29 A view of the retina as seen with an ophthalmoscope. (a) A photograph and (b) an illustration of the optic fundus (back of the eye). Optic nerve fibers leave the eyeball at the optic disc to form the optic nerve. (Note the blood vessels that can be seen entering the eyeball at the optic disc.)

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Responses

  • Eberardo Lucchesi
    Is the sulcus located in the anterior or posterior chamber?
    7 years ago
  • T Burrows
    How does the vitreous chamber differ from the anterior and posterior chambers?
    7 years ago
  • tapani
    How does the vitreous body differ from the aqueous humor in location and viscosity?
    7 years ago
  • BORIS
    What is circularly and radially arranged pigmented smooth muscle fibers?
    7 years ago
  • ELIZA HUGHES
    What attaches to the suspensory ligament and regulates lens thickness?
    7 years ago
  • MARCO
    Where are the fovea centralis in the eyeball?
    7 years ago
  • uta
    Where does the aqueous humor form?
    7 years ago
  • renee
    What does posterior chamber consist?
    7 years ago
  • abdullah omar
    Where is the fovea centralis located?
    7 years ago
  • bell
    Where aqueous humor forms?
    6 years ago
  • panu
    What refracts light and focuses onto fovea centralis?
    5 years ago
  • irene
    What secrets the vitreous and aqueuos humour in human eye?
    2 years ago

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