The Eye ACCESSORY STRUCTURES

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1 The Eye The eye forms a visual image and projects it onto the sensory receptors (photoreceptors) of the retina. ACCESSORY STRUCTURES Anatomy and Physiology Text and Laboratory Workbook, Stephen G. Davenport, Copyright 2006, All Rights Reserved, no part of this publication can be used for any commercial purpose. Permission requests should be addressed to Stephen G. Davenport, Link Publishing, P.O. Box 15562, San Antonio, TX, Accessory Structures The accessory structures of the eye include the (1) eyebrows, (2) eyelids, (3) eyelashes, (4) conjunctiva, (5) lacrimal apparatus, and the (6) extrinsic eye muscles. Accessory Structures Eyebrows The eyebrows protect the eyes by (1) providing shade, and (2) they direct the movement of perspiration from the forehead away from the eyes. Eyelids (palpebrae) The eyelids, or palpebrae, (1) protect the eyes from foreign objects and (2) keep the eyes from drying by spreading lacrimal and other secretions and by covering the eyes. The inner surface of the eyelid is covered with a mucous membrane called the palpebral conjunctiva. Internally, the eyelids house the roots of the eyelashes and sebaceous glands. Large modified sebaceous glands of the eyelids are the Meibomian glands. The Meibomin glands proudce an oil-like secretion that functions to keep the eye moist. Upon blinking, the oily secretion spreads to the anterior surface of the eye where it slows evaporation. Accessory Structures Figure 18.1 The eyelids, or palpebrae, (1) protect the eyes from foreign objects and (2) keep the eyes from drying by spreading lacrimal and other secretions and by covering the eyes. This scanning power photograph of the eyelid shows the structure of the eyelid. Internally, the eyelids house the roots of the eyelashes and sebaceous glands. Large modified Eyelashes The eyelashes (1) protect the eyes from foreign objects and (2) help shade the eyes. Conjunctiva The conjunctiva is a mucous membrane which covers (1) the inner aspect of the eyelid (palpebral conjunctiva) and (2) the anterior surface of the eye (ocular, or bulbar, conjunctiva) except the cornea. The conjunctiva is thin and transparent. Beneath the surface of the ocular conjunctiva, small blood vessels and the white portion of the eye (sclera) can be observed. The conjunctiva functions to (1) protect the eye by providing a site for sensory receptors (pain) and (2) produces lubricating mucus. 1

2 Figure 18.2 The accessory structures of the eye include the eyebrows, eyelids, eyelashes, conjunctiva, lacrimal apparatus, and the extrinsic eye muscles. Accessory Structures Lacrimal apparatus Each eye has a lacrimal apparatus. A lacrimal apparatus consists of the (1) lacrimal gland and (2) the structures which drain the secretions (tears) from the lacrimal apparatus. Each lacrimal gland is located superiorly and laterally to each eyeball. Lacrimal secretions (tears) from a lacrimal gland flow onto the upper conjunctiva through several small lacrimal ducts. Tears pass medially over the anterior surface of the eyeball and enter two small openings (lacrimal puncta), one located at each medial margin of each eyelid. Each lacrimal punctum opens into a lacrimal canal, which drains into the lacrimal sac. Each lacrimal sac drains into a nasolacrimal duct, which enters the nasal cavity at the inferior nasal meatus (chamber under the bone called the inferior nasal concha). Accessory Structures Figure 18.3 A lacrimal apparatus consists of the (1) lacrimal gland and (2) the structures which drain the secretions (tears) from the lacrimal apparatus. Extrinsic eye muscles The six muscles which move the eyeball are called the extrinsic eye muscles. Four of the six muscles are named rectus muscles, and the two other muscles are named oblique muscles. The rectus muscles are further named by their position as the (1) superior rectus, (2) inferior rectus, (3) medial rectus, and (4) lateral rectus muscle, and the oblique muscles are named the (5) superior oblique and the (6) inferior oblique muscle. Figure 18.4 The six muscles which move the eyeball are called the extrinsic eye muscles. Four of the six muscles are named rectus muscles, and the two other muscles are named oblique muscles. Figure 18.5 The six extrinsic muscles of the eye function in movements of the eye as shown in the photograph of the preserved sheep eye. 2

3 EYE MOVEMENTS Figure 18.6 Lateral view of the eye showing the movements directed by the extrinsic muscles. Eye Movements The medial and lateral rectus muscles function in eye movements in the horizontal plane. Because the medial and lateral rectus muscles are inserted along the horizontal axis of the eye, their contraction produces movement along the horizontal axis. The medial rectus muscle functions in eye adduction (movement of the eye toward nose) and the lateral rectus muscle function in eye abduction (movement of the eye away from nose). Eye Movements The remaining four muscles, the superior and inferior rectus muscles and the superior and inferior oblique muscles, function in movement of the eye in the vertical plane moving the eye upward (elevation) and downward (depression). Because the superior and inferior rectus muscles are not inserted along the vertical axis of the eye, their contractions do not produce purely vertical movements (elevation and depression). When the eye is abducted, the superior and inferior rectus muscles exert the movements of elevation and depression, respectively. When the eye is fully adducted the superior and inferior oblique muscles exert the movements of depression and elevation, respectively. When the eye is in its forward position, all four muscles make contributions to elevation and depression. Structure of the Eyeball STRUCTURE OF THE EYEBALL The eyeball, which is mostly spherical in shape, is housed within and is protected by the bony orbit. Only a small portion of its anterior aspect is exposed to the external environment. Externally, the eye is surrounded by protective adipose tissue. Six muscles, the extrinsic muscles of the eye, control the movement of the eye. Internally, the eye is filled with fluids (humors) and is divided by the lens into an (1) anterior cavity (segment) - contains aqueous humor (2) posterior cavity (segment) - contains the vitreous humor (body). 3

4 Structure of the Eyeball Wall of the Eye The wall of the eye consists of three layers (tunics). From outer to inner, they are the (1) fibrous tunic, the (2) vascular tunic, and the (3) sensory tunic. Structure of the Eyeball Wall of the Eye Fibrous tunic The fibrous tunic is the layer of tough dense connective tissue that surrounds the eye. The two components of the fibrous tunic are the sclera and the cornea. Vascular tunic The vascular tunic contains numerous blood vessels (vascular), pigments, and the intrinsic muscles of the eye. The components of the vascular tunic are the choroid, ciliary body, and iris. The intrinsic muscles of the eye describe the muscles found within the eye and include the muscles of the ciliary body, the ciliary muscle, and the muscles of the iris. Neural tunic The neural tunic is the inner layer of the eye and is the retina. The retina consists of an inner neural layer and an outer pigmented layer. Figure 18.7 The general surface anatomy of the eye is shown. The extrinsic muscles are the six ocular muscles that move the eye. Figure 18.8 A drawing of the eye as seen in a horizontal section. Figure 18.9 The general anatomy of the eye as demonstrated with a horizontal section of a preserved sheep eye. Common abnormalities shown is a preserved eye are the presence of a translucent (or opaque) cornea and lens, and a retina that shows surface wrinkling. Figure A drawing of the eye as seen in horizontal and front sections. 4

5 Lab Activity 2 Structure of the Eye Observe a microscope slide preparation of the eye and/or the following illustrations and photographs. The label on the slide will usually identify the section, such as general section, or showing the optic nerve, and/or fovea centralis, etc. Figure Horizontal meridional section of the monkey eye showing both the fovea centralis and the optic disc. Sclera FIBROUS TUNIC The outer fibrous tunic consists of the (1) sclera and the (2) cornea. The sclera is the tough, opaque white portion of the eyeball formed by fibrous connective tissue. It completely surrounds the eyeball except for two locations: (1) anteriorly, where it merges with the cornea and (2) posteriorly, where it is pierced by the optic nerve. The sclera functions in (1) providing an attachment site for the extrinsic muscles, (2) gives the eye shape, and (3) provides protection. Figure The fibrous sclera forms the outer portion of the wall of the eye except anteriorly, where it merges with the (1) cornea and (2) posteriorly, where it is pierced by the optic nerve. Cornea The cornea primarily functions in (1) allowing light into the eye, (2) in focusing (refraction) of light, and (3) protection. The cornea is formed from fibrous connective tissue and is continuous with the sclera, the corneal-scleral junction is called the limbus. The cornea is the transparent anterior window of the eyeball and is the eye s primary structure for focusing incoming light. The external surface of the cornea is lined with stratified squamous epithelium (not the conjunctiva). Numerous free nerve endings, which primarily function as pain receptors, are located in the corneal epithelium. Internally, the cornea is formed from layers of transparent collagen fibers, collectively called the stroma. 5

6 VASCULAR TUNIC Figure The cornea is the transparent anterior window of the eye and is the eye s primary structure for focusing incoming light. Identify the middle vascular tunic, which consists of the (1) choroid, (2) ciliary body, and (3) iris. Choroid The choroid is the highly vascular posterior portion of the vascular tunic. Its numerous blood vessels (1) supply nutrients to the fibrous and sensory (retina) tunics, and (2) its pigments absorb light. Anteriorly, the choroid joins the ciliary body near the anterior margin of the retina, the ora serrata. Figure The choroid is the highly vascular posterior portion of the vascular tunic. Ciliary Body The ciliary body is a region of the vascular tunic anterior to the choroid. It consists of the (1) ciliary muscle and the (2) ciliary processes. Ciliary muscle The ciliary muscle is a ring of smooth muscle that functions in the regulation of the shape of the lens. The epithelial surface of the ciliary muscle is modified into folds called ciliary processes. Ciliary processes The ciliary processes are folds formed from the epithelium that covers the ciliary muscle. The ciliary processes (1) secrete the fluid (aqueous humor) of the anterior cavity (segment) and (2) provide attachment sites for the suspensory ligament, which attaches to the lens. Suspensory Ligament The suspensory ligament extends from the ciliary processes to the lens. It consists of fibers that (1) provide for positioning of the lens and for (2) the transfer of tension produced by the ciliary muscle in the regulation of the shape of the lens. 6

7 Iris and Pupil Figure The ciliary body consists of the ciliary muscle and the ciliary processes. Iris The iris is the most anterior portion of the vascular tunic. The iris extends anteriorly from the ciliary body and divides the anterior cavity (segment) into the anterior (in front of iris) and posterior (behind the iris) chambers. The iris regulates the amount of light that enters the eye. Two groups of smooth muscle fibers, the dilator fibers and the sphincter (constrictor) fibers, control the diameter of the pupil, the central opening in the iris. The dilator fibers, which dilate, or increase the diameter of the pupil, are controlled by the sympathetic division of the autonomic nervous system (ANS). The sphincter (constrictor) fibers, which decrease the diameter of the pupil, are controlled by the parasympathetic division of the ANS. Pupil The pupil is the central opening in the iris. SENSORY TUNIC (RETINA) Figure The iris extends anteriorly from the ciliary body and divides the anterior cavity (segment) into the anterior (in front of iris) and posterior (behind the iris) chambers. The sensory tunic, or retina, is the inner tunic of the eye. Sensory Tunic The retina consists of the (1) inner neural (nervous) layer and an (2) outer pigmented layer. The neural layer of the retina consists of three major groups of cells. From outer to inner the cell layers are the (1) photoreceptors, (2) bipolar cells, and (3) the ganglion cells. Figure The neural layer of the retina consists of three major groups of cells. 7

8 Neural Layer Seven Layers A more detailed description of the retina divides the neural portion of the retina into seven layers. From outer to inner the layers are the (1) rods and cones, (2) outer nuclear layer, (3) outer plexiform layer, (4) inner nuclear layer, (5) inner plexiform layer, (6) ganglion cell layer, and (7) the layer of optic nerve fibers. Figure A detailed description of the retina divides the neural portion of the retina into seven layers. Figure Detailed description of the retina of the monkey eye. Sensory Tunic Plexiform Layers The plexiform layers are regions that contain nerve fibrils (axons and dendrites) and synapses. Retina Outer Layer The outermost layer of the neural retina, the layer of rods and cones. This layer contains the light receptive elements of the photoreceptors that are either rod or cone shaped. The nuclei of the photoreceptors are located in the outer nuclear layer of the retina. The photoreceptors terminate with synapses to the bipolar cells in the outer plexiform layer. Retina Retina Outer Layer The rods do not respond to different wavelengths of light (color), thus, are only responsible for producing images without color (black and white). Rods also perform best under low light conditions. Cones function in color reception and perform best under higher light than allowed for rods. There are three varieties of cones, (1) red sensitive cones, (2) green sensitive cones, and (3) blue sensitive cones. Even though each cone is most sensitive in its named wavelength, overlapping sensitivity produces responses for the full color spectrum. Figure The outermost layer of the neural retina consists of the rods and cones. Named by their shape, rods do not result in color discrimination. Color discrimination is produced by the overlapping sensitivities of the three types of cones, the red cones, the blue cones, and the green cones. 8

9 Bipolar and Ganglion Cells Bipolar Cells Bipolar cells are neurons that have two processes associated with their cell body. Their receptive portions (dendrites) are found in the outer plexiform layer associated with the photoreceptors (and amacrine cells). The bipolar cells terminate with synapses to the ganglion cells in the inner plexiform layer. Ganglion Cells The receptive portion of the ganglion cells is in the inner plexiform layer where they synapse with bipolar cells (and horizontal cells). The axons of the ganglion cells form the layer called the ganglion nerve fibers (ganglion axons ) that converge at the optic disc to form the optic nerve. Macula Lutea The macula lutea is a region of the retina which contains only cones. The fovea centralis is located at the center of the macula lutea. Ophthalmologists and opticians routinely examine the macula lutea for pathology during eye examinations. An ophthalmoscopic view of the macula lutea reveals it to be a dark area lateral to the optic disc, the exit point of the optic nerve. Fovea Centralis The fovea centralis is a small pit located at the center of the macula lutea. In humans the fovea centralis contains only cones and is the area of most acute vision. The visual axis, the focal point of light through the eye, falls directly upon the fovea centralis. Figure A photograph of the back of the eye as seen through an opthalmoscope. Figure A scanning power view of the fovea centralis (and wall) of the monkey eye. Figure The fovea centralis is a small pit located at the center of the macula lutea. The fovea centralis is the area of most acute vision. The visual axis, the focal point of light through the eye, falls directly upon the fovea centralis. 9

10 Optic Disc (Blind spot) The optic disc consists of axons of the ganglion cells (layer of nerve fibers) that converge and exit the eye as the optic nerve. The optic disc is called the blind spot because it does not contain photoreceptors (retina is absent at this location). Figure Low and high power views of the fovea centralis (nonhuman). Optic Disc (Blind spot) Figure The optic disc consists of axons of the ganglion cells (layer of nerve fibers) that converge and exit the eye as the optic nerve Ophthalmologists and opticians routinely examine the optic disc for pathology during eye examinations. The normal optic disc is observed as a white area with a slight depression called the optic cup. Also seen at the optic disc are the central artery and vein, two major blood vessels that vascularize a major portion of the retina by routing through the optic nerve. The outer retina (photoreceptors) receive most of their vascular supply from the choroid, and the inner retina (bipolar and ganglion cells) are vascularized by the central artery and vein. SEGMENTS AND CHAMBERS Figure The optic disc as seen through an ophthalmoscope. The normal optic disc is observed as a white area with a slight depression called the optic cup. Also, seen at the optic disc are the central artery and vein. 10

11 Segments and Chambers Anterior cavity (segment) The anterior cavity (segment) is the cavity anterior to the lens. It contains aqueous humor and is divided by the iris into the anterior and posterior chambers. Anterior chamber The anterior chamber is the division of the anterior cavity that is anterior to the iris (and posterior to the cornea). It contains aqueous humor. Posterior chamber The posterior chamber is the division of the anterior cavity that is posterior to the iris (and anterior to the lens). It contains aqueous humor. Segments and Chambers Posterior cavity (segment) The posterior cavity (segment) is the cavity located posterior to the lens. The posterior cavity contains the fluid called vitreous humor (body). Because the vitreous humor is firm and gelatinous, the term body is frequently used to replace humor (fluid). AQUEOUS HUMOR Location, production, and reabsorption Figure The anterior and posterior chambers are named in reference to the iris. Both chambers contain aqueous humor. Aqueous Humor The aqueous humor is the fluid in the anterior cavity (segment). Aqueous humor is produced at the ciliary processes by capillary filtration. From the ciliary process, the aqueous humor flows into the posterior chamber, then passes through the pupil into the anterior chamber. From the anterior chamber, aqueous humor enters into the scleral venous sinus (canal of Schlemm) at the inner junction of the sclera and the cornea, then enters venous circulation. Figure The aqueous humor is the fluid in the anterior cavity (segment). It is produced at the ciliary processes by capillary filtration and reabsorbed at the scleral venous sinus. 11

12 Lens LENS The lens is a biconvex structure formed of layers of cells called lens fibers. The lens functions in the focusing of light onto the retina. The lens is attached to the suspensory ligament, which transfers tension from the ciliary muscle to the capsule of the lens. Contraction and relaxation of the ciliary muscle function in the regulation of the shape (curvature) of the lens. Changing the curvature of the lens changes its refraction (bending light), thus allowing focusing of light onto area of the retina of acute vision, the fovea centralis. DISSECTION OF THE EYE - (SHEEP) Figure The lens is a biconvex structure formed of layers of cells called lens fibers. The lens functions in the focusing of light onto the retina. EXTERNAL ANATOMY 12

13 Figure A sketch of the eye showing its general external anatomy. Figure The preserved sheep eye. Usually, specimens are not supplied with eyelids and eyelashes. A large quantity of adipose tissue normally surrounds and protects the eye. Eyelashes (usually not present on dissection specimens) Eyelashes are the fringe of hairs that extend across the edge of the eyelid. Eyelashes function to protect the eye from the entrance of foreign substances such as sweat. Acting like levers, eyelashes enhance touch and promote the blink reflex. Figure The anterior surface of a sheep eye after the removal of most of the adipose tissue to expose the extrinsic eye muscles. Some preserved eyes have a translucent (permits passage of light but image is blurred) cornea, which allows the identification of the iris. Usually, the cornea is opaque (does not transmit light). Eyelids (usually not present on dissection specimens) The two eyelids (palpebrae) are folds of skin and muscle that can cover the anterior surface of the eye. The inner surface of the eyelids is covered with a mucous membrane, the palpebral conjunctiva. The eyelids house the eyelashes at their margins and contain numerous sebaceous glands, the Meibomian glands. The eyelids function to protect the eye from drying and from damage from foreign substances. Cornea The cornea of preserved eyes always appears cloudy. Depending upon the specimen, the cornea may be slightly translucent, thus, the iris may be observed beneath the cornea s surface. However, the cornea is usually totally opaque on preserved eyes. In the normal eye, the cornea is the transparent anterior window of the eye. The cornea is the eye s primary structure for focusing incoming light. The cornea is formed from fibrous connective tissue and is continuous with the sclera, the corneal-scleral junction is called the limbus. The external surface of the cornea is lined with stratified squamous epithelium (not the conjunctiva). The cornea primarily functions in (1) allowing light into the eye, (2) in focusing (refraction) of light, and (3) protection. 13

14 Conjunctiva The conjunctiva is a mucous membrane which covers (1) the inner aspect of the eyelid (palpebral conjunctiva) and (2) the anterior surface of the eye (ocular, or bulbar, conjunctiva) except the cornea. The conjunctiva is thin and transparent. Beneath the surface of the ocular conjunctiva, small blood vessels and the white portion of the eye (sclera) can be observed. The conjunctiva functions to (1) protect the eye by providing a site for sensory receptors (pain) and (2) produces lubricating mucus. Sclera The sclera is the tough, opaque white portion of the eyeball formed by fibrous connective tissue. It completely surrounds the eyeball except for two locations: (1) anteriorly, at the location of the cornea and (2) posteriorly, where it is pierced by the optic nerve. The sclera functions in (1) providing an attachment site for the extrinsic muscles, (2) gives the eye shape, and (3) provides protection. Extrinsic eye muscles The six muscles which move the eye are collectively called the extrinsic eye muscles. The extrinsic muscles are surrounded by adipose tissue and are identified by their reddish color and firm texture. Four of the six muscles are named rectus muscles, and the two other muscles are named oblique muscles. The rectus muscles are further named by their position as the (1) superior rectus, (2) inferior rectus, (3) medial rectus, and (4) lateral rectus muscle, and The oblique muscles are named the (5) superior oblique and the (6) inferior oblique muscle. Figure The six muscles which move the eye are collectively called the extrinsic eye muscles. The extrinsic muscles are surrounded by adipose tissue and are identified by their reddish color and firm texture. Optic Nerve The optic nerve originates at the convergence of retinal (ganglion cells) axons at the posterior wall of the eye. The optic nerve is slightly medial to the visual axis and extends slightly superiorly to exit the orbit of the eye at the optic foramen of the sphenoid bone. Usually, on preserved specimens the optic nerve is cut short and is seen as a white rodlike stub. Figure The optic nerve originates at the convergence of retinal (ganglion cells) axons at the posterior wall of the eye. The optic nerve is usually cut short and is seen as a white rod-like stub. 14

15 INTERNAL ANATOMY Figure Section the eye in a frontal plane to make approximately equal halves, an anterior portion and a posterior portion. Vitreous Humor ANTERIOR PORTION OF THE CUT EYE Position the anterior portion of the eye so that you are observing its interior. The vitreous humor (body) is the clear gelatinous substance that occupies the posterior cavity (segment), the space between the lens and the retina. Function of the vitreous humor include (1) giving the eye shape, (2) protecting the eye by absorbing shock, and (3) supporting the retina against the wall of the eye. STEP 1 CAREFULLY REMOVE THE VITREOUS HUMOR Figure A sketch of the anterior portion of the dissected sheep eye as seen immediately after sectioning. The vitreous humor (body) has been removed. 15

16 Lens In the normal eye, the lens is a crystal clear elastic biconvex structure formed of layers of cells called lens fibers. In the preserved eye the lens is hard and opaque. The lens functions in the focusing of light onto the retina. It is attached to the suspensory ligament, which transfers tension from the ciliary muscle for the regulation of shape (curvature) of the lens. Changing the curvature of the lens changes its refraction (bending light), thus allowing focusing of light onto area of the retina of acute vision, the fovea centralis. The suspensory ligament is too small to be observed on the dissection. Figure The anterior portion of the dissected sheep eye as seen immediately after sectioning. The vitreous humor (body) has not been removed. Ciliary Body The ciliary body, is a region of the vascular tunic anterior to the choroid. It consists of the (1) ciliary muscle and the (2) ciliary processes. Ciliary muscle The ciliary muscle is a ring of smooth muscle that functions in the regulation of the shape of the lens. The epithelial surface of the ciliary muscle is modified into folds called ciliary processes. Ciliary processes The ciliary processes are folds formed from the epithelium that covers the ciliary muscle. The ciliary processes (1) secrete the fluid (aqueous humor) of the anterior cavity (segment) and (2) provide attachment sites for the suspensory ligament. CUT WALL OF THE EYE The wall of the eye that surrounds the posterior cavity (segment) consists of three layers, from inner to outer, the (1) retina, (2) choroid, and (3) the sclera. Retina Observe the inner layer, the retina. In the preserved eye the retina is thin, translucent, and detached forming numerous wrinkles. The retina functions as the neural layer of the eye, converting the energy of light into electrical energy. Choroid Carefully pull the retina away from its overlying layer, the choroid. The choroid is dark (purple-black) in color. The functions of the choroid include (1) supplying a portion of the retina s blood supply, and (2) absorbing light. A specialized region of the choroid seen in the posterior portion of the cut eye is the tapetum lucidum, an iridescent blue region that reflects light back to the retina. The tapetum lucidum is present in animals with eyes modified for enhanced night vision. 16

17 Sclera Carefully pull the choroid away from its overlying layer, the sclera. The sclera is the white outer layer of the eye s wall. The sclera is formed of fibrous connective tissue. The functions of the sclera include (1) protection and (2) giving the eye shape. STEP 2 REMOVE THE LENS FROM THE CILIARY BODY Figure Remove the lens from the ciliary body to expose the posterior surface of the iris and the cornea. Figure A translucent lens from a freshly preserved sheep eye is shown placed on text. The biconvex lens converges light rays and produces an image that is magnified. Iris The iris is the most anterior portion of the vascular tunic. Removing the lens from the ciliary body exposes its darkly pigmented posterior surface. The pupil is the central opening in the iris. The posterior surface of the cornea is observed through the pupil. The function of the iris is to regulate the amount of light entering the eye. Constriction and dilation of the muscles of the iris produces a corresponding constriction and dilation of the pupil. ANTERIOR CAVITY (SEGMENT) 17

18 Anterior Cavity Removal of the lens exposes the anterior cavity, the cavity anterior to the lens. It contains aqueous humor and is divided by the iris into the anterior and posterior chambers. Anterior chamber Observe the anterior chamber, the division of the anterior cavity that is anterior to the iris and posterior to the cornea. The anterior chamber contains aqueous humor. Posterior chamber Observe the posterior chamber, the division of the anterior cavity that is posterior to the iris and anterior to the lens. The location of the lens on the eye specimen is observed by the margin of the ciliary body. The anterior chamber contains aqueous humor. Figure Sketch of the anterior portion of the eye after removal of the lens. STEP 3 REMOVE THE IRIS AND CILIARY BODY Figure Removal of the lens from the ciliary body exposes the posterior surface of the iris, the pupil, and the posterior surface of the cornea. Removal of Iris and Ciliary Body Observe the posterior surface of the portion remove with the forceps and identify the (1) ciliary body, (2) posterior surface of the iris, (3) pupil, (4) retina, and (5) choroid. Rotate the specimen to expose its anterior surface and identify the anterior surface of the (1) iris, (2) pupil, and (3) choroid. Figure Grab the ciliary body and the iris with forceps and pull the structures out of the anterior portion of the eye. 18

19 Figure The portion removed with forceps reveals the details of the ciliary body and iris. Rotation of the specimen reveals the anterior surface of the iris Figure The cornea and sclera form the outer fibrous tunic of the eye. POSTERIOR PORTION OF THE CUT EYE STEP 1 CAREFULLY REMOVE THE VITREOUS HUMOR Position the posterior half of the eyeball so that you are observing its interior. Retina Figure The vitreous humor (body) is a gelatinous substance that occupies the posterior cavity, the area posterior to the lens. After removal of the vitreous humor, identify the retina. The retina functions as the neural layer (tunic) of the eye, converting light energy into electrical energy, nerve impulses. On the eye specimen, the retina usually appears as a thin wrinkled (detached) thin membrane. Notice that the retina is firmly attached to a white round structure, the optic disc. The optic disc is the point where retinal axons converge and pierce the wall of the eye as the optic nerve. The optic disc is also called the blind spot as it is void of the retina. 19

20 Choroid Observe the choroid, the middle pigmented vascular layer of the wall of the eye. The choroid functions to absorb light and through its numerous blood vessels is a vascular supply for the sclera and retina. The choroid is located directly outside of the retina. Probe the retina away from the pigmented choroid. A portion of the choroid, the tapetum lucidum, is an iridescent blue. The tapetum lucidum is found in animals adapted for night vision (not humans) as it reflects light back onto the retina. Figure A sketch of the posterior portion of the cut eye, with the vitreous humor (body) removed. The layers of the wall of the eye are sectioned to show their positions. PHYSIOLOGY OF VISION Figure The posterior portion of the cut eye, with the vitreous humor (body) removed. The layers of the wall have been separated to show their positions. Light LIGHT Light is a form of energy, electromagnetic radiation, that is transmitted in waves. Light is a form of energy, electromagnetic radiation, that is transmitted in waves. The energy of light is called a photon, thus, light is considered to be a transmission of photons in waves. The waves of visible light are of different lengths (wavelengths), and if separated as with a prism, produce a color spectrum. 20

21 Light Figure Photograph of the visible spectrum projected by a prism. A prism separates light by its wavelength and forms the color spectrum. The visual receptors (photoreceptors) of the eye, the rods and cones, are sensitive to the portion of the electromagnetic spectrum called visible light. Usually, light enters the eye as reflected light. Reflection of light is the return (or bouncing) of light from a surface. When light strikes an object, the object may reflect all wavelengths, absorb some, or absorb all of the wavelengths. The light waves that are reflected from the object give the object its color. For example, a red object is seen as red because the object absorbs all the wavelengths except red. The red wavelengths are reflected and strike the color photoreceptors (red sensitive cones) of the retina. White results when all waves are reflected from an object and black results when all waves are absorbed by an object. Light and Cones The color receptors of the retina are the cones. Three color sensitive cones are found in the retina, (1) red sensitive, (2) green sensitive, and (3) blue sensitive and The cones function in the interpretation of the additive primary colors, red, blue and green (RGB). The ability to determine color relies on the amount of stimulation of the three different types of color cones. Figure Three types of cones, red sensitive, blue sensitive, and green sensitive are responsible for the perception of color. Each cone is most sensitive in its named region of the color spectrum. The amount of stimulation of each of the three types of cones gives the ability to detect light in the full visible spectrum. Lab Activity 4 Color Blindness Test Color blindness is the inability to distinguish certain or all colors of light. Color blindness results because of one, two, or all color sensitive cones being nonfunctional. The most common type of color blindness is red/green color blindness and is more common in males than females (the genes for the red/green pigments are located of the X chromosome). Obtain an Ishihara Test Chart Book and perform the color blindness exam. Depending upon the test books available, the test is usually for total color blindness and/or red/green color blindness. The test is best performed in a daylight illuminated room. Typically, the test discriminates color blindness by using different dyes on each plate. Thus, when plates are made by using more than one dye, color blind subjects identify different numbers on the same plate. Figure Color blindness test charts are typically made by using more than one dye in the production of the numbers on each test chart, thus, depending upon the type of color blindness, subjects will identify different numbers. Shown in this figure are three test plates, each with a number 70 in a primary color corresponding to the three most sensitive cones, blue, red, and green. 21

22 Light Refraction LIGHT REFRACTION Light travels in a straight line and when reflected from an object travels away from the object in many directions. A primary function of the eye is to bend (refract) light so that the light (image) falls in sharp focus on the retina s area of most acute vision, the fovea centralis. Light is bent, or refracted, when it is deflected from a straight path as it passes obliquely from one transparent medium into another transparent medium and its speed (velocity) changes. For example, when light passes obliquely from air into glass, its velocity changes and the light is directed into another straight line. Typical structures used to refract light are lenses. The curved surfaces of lenses are the points of refraction. The more the surface is curved the more the light is refracted. Plates of glass with flat surfaces, such as windows, do not refract light. Two common refractive lenses are convex and concave lenses. Convex Lens Figure Refraction occurs when light passes obliquely from one transparent medium into another transparent medium. Two areas of refraction are shown in the photograph of a straw in a glass of water. One area of refraction is at the surface of the water (as light passes from air to water) and the other as at the side (curved surface) of the glass (as light passes from air-glass-water). Convex lenses are lenses that are thicker in the middle than at their periphery. Convex lenses are commonly called converging lenses as they function to refract light inward. They converge light from a distant source and focus it to a point, commonly called a focal point (length). Increasing lens convexity, increases the refraction of the lens. The greater the refraction of the lens, the shorter the focal length. The shorter the focal length the stronger the lens. Concave Lens A concave lens has the opposite effect of a convex lens. Concave lenses are thicker at their periphery than in the middle. Concave lenses are commonly called diverging lenses as they function to refract light outward, and do not form a focal point. Figure A convex lens is commonly called a converging lens. A convex lens converges light to a point, commonly called a focal point (length). 22

23 LIGHT REFRACTION ONTO THE RETINA Figure A concave lens is commonly called a diverging lens. A concave lens refracts light outward. Refraction onto Retina The two major refractory structures of the eye are the cornea and the lens. Cornea The cornea is the primary refractory structure of the eye, with most refraction occurring as light enters at its anterior surface. The cornea converges light rays inward where they are further refracted as they pass through the anterior and then posterior surfaces of the lens. Lens The lens functions to dynamically focus light (by changes in its shape) onto the fovea centralis of the retina. The shape of the cornea, thus, its refraction does not change. Accommodation Accommodation is the ability of the eye to adjust its focal length and is achieved by the ciliary body and the lens. Changes in focal length are necessary to adjust incoming light waves to fall upon the fovea centralis. The shape of the lens, thus its focal length, is changed by the contraction and relaxation of the ciliary muscle. The outer elastic covering of the lens, the lens capsule, is attached to the ciliary muscle by the suspensory ligament. Changes in tension produced by the contraction and relaxation of the ciliary muscles changes the shape (convexity) of the lens. Accommodation Distant Objects Light from distant objects enters the eye as nearly parallel waves and do not require the eye to undergo accommodation. When the ciliary muscles are relaxed, the eye is accommodated (focused) for the far point of vision. The normal human eye has a far point of vision of 20 feet. Thus, viewing an object at 20 feet or more does not require the eye to accommodate. Accommodation Near Objects Light from near objects (closer than 20 feet) requires eye accommodation because the light waves are not as parallel. The closer the image the less parallel the light waves and the more the eye must accommodate. With decreasing distance from the eye, the ciliary muscle contracts more and more, pulling inward toward the lens, thus, reducing tension upon the suspensory ligament. With reduced tension, the elastic capsule of the lens recoils and the lens becomes more convex (thicker in the center) producing a shorter focal length. The shortest distance at which the eye can focus is the near point of vision. 23

24 VISUAL ACUITY Figure At the left of the illustration, the ciliary muscle of the dissected sheep eye is shown to illustrate the resulting increasing lens convexity resulting from the contraction of the ciliary muscle. At the right, a sectional sketch illustrates the necessity of changing lens convexity to focus light onto the fovea centralis of the retina. Visual Acuity Visual acuity is a person s sharpness of visual detail. The eye that produces normal visual acuity (sharp focus at 20 feet) is described as emmetropic. Abnormal visual acuity results from abnormal structural features of the eye that cause the visual axis (image) not be focused on the fovea centralis of the retina at 20 feet. Twenty feet is used to describe and to test for visual acuity because it is the distance where the lens is not accommodated (shape changed by the activity of the ciliary muscle). Common abnormal structural features include the abnormally shaped eyeball, such as having either a too short or too long eyeball, or an eyeball that has either an abnormally curved lens and/or cornea. Two common visual abnormalities that involve the abnormally shaped eyeball are hyperopia and myopia. A common visual abnormality that involves the abnormally shaped cornea and/or lens is astigmatism. Hyperopia Hyperopia, or farsightedness, results when the eyeball is too short and the point of focus is behind of the fovea centralis. Individuals with hyperopia can focus on distant objects but the eye cannot focus on near objects. If eyeglasses are prescribed as the solution for hyperopia, a converging lens, such as a convex lens, is the type of lens used. Myopia Myopia, or nearsightedness, results withe the eyeball is too long and the point of focus is in front of the fovea centralis. Individuals with myopia can focus on near objects but the eye cannot focus on far objects. If eyeglasses are prescribed as the solution for myopia, a diverging lens, such as a concave lens is the type of lens used. ASTIGMATISM 24

25 Astigmatism Astigmatism is a visual condition that results when light rays are not focused evenly onto the retina because of an irregular curvature of the cornea and/or lens. If the cornea is described as perfectly dome shaped, like half of a basketball, then a corneal astigmatism might be described as the cornea in an oblong shape like half a football (football is sectioned along its length). A nonsymmetrical shape can produce more than one focal point, resulting in an image that is blurred and distorted. If eyeglasses are prescribed as the solution for astigmatism, a lens is prescribed with a corrective area to match the area of the cornea and/or lens that is astigmatic. Figure A perfectly symmetrical cornea produces only one focal point. A cornea that is elongated in one axis and compressed in the other axis produces two focal points. Figure Common abnormal structural features include the abnormally shaped eyeball, such as having either a too short or too long eyeball, or an eyeball that has either an abnormally curved lens and/or cornea. Two common visual abnormalities that involve the abnormally shaped eyeball are hyperopia and myopia. A common visual abnormality that involves the abnormally shaped cornea and/or lens is astigmatism. VISUAL TESTS Visual Acuity Visual acuity is measured by the person s ability to read print of a certain size at a distance of 20 feet. The test distance of twenty feet is used as a standard because it is at this distance the lens of a normal eye (emmetropic) does not change shape for focusing (point of far vision). The visual acuity test compares what the test subject can read at the standard test distance of 20 feet to what a person with normal vision reads at the distance specified on the test chart. A visual acuity measurement is written to the side of each line on the test chart. The top number of the visual acuity reading is the test distance, and will always be 20 feet. The bottom number is the distance at which a person with normal vision, emmetropic, reads the line. Visual Acuity For example, if you can read to the line that specifies 20/100, this means that you read at 20 feet (standard test distance) what a person with normal vision reads at 100 feet. Interpreting this reading indicates that you are nearsighted (myopic). If your visual acuity is normal (emmetropic), you can read the test print at the test distance of 20 feet that a person with normal vision can read at 20 feet. Thus, your vision is 20/20. If you can read the letters specified for 20/10 feet, then you read at 20 feet what a person with normal vision can read at 10 feet. This reading would indicate that you are slightly farsighted (hyperopic). 25

26 Figure The visual acuity screening test uses a standard test distance of 20 feet and print of different sizes to determine visual acuity. The top number of the visual acuity reading is the test distance, and will always be 20 feet. The bottom number is the distance at which a person with normal vision, emmetropic, reads the line. Lab Activity 5 Test for Visual Acuity Procedure In a well-illuminated room stand 20 feet from the visual acuity screening test page (if a test page is not available, the above illustration can be used). \ Start with the top of the test page, which normally indicates 20/200 and read the lines downward. Have someone verify your results. Test one eye at a time by holding a cover in front of the other eye. If you wear eyeglasses, first test your eyes with your glasses and then without. If you wear contacts, test your eyes with them. Results Record in the worksheet the visual acuity reading for the last line that you could accurately read. Lab Activity 6 Test for Astigmatism Astigmatism can be tested by observing a test card which consists of converging radial lines. If no astigmatism exists, all of the radial lines will be of equal sharpness. However, if astigmatism exists, lines will be distorted in areas that correspond to the unequal curvature of the cornea and/or lens Lab Activity 6 Test for Astigmatism Procedure In a well-illuminated room, stand about three feet away from an astigmatism screening illustration (if one is not available use the above illustration). Test one eye at a time by holding a cover in front of the other eye. Look directly at the center of the illustration. If you wear eyeglasses, first test your eyes with your glasses and then without. If you wear contacts, test your eyes with them. Results An astigmatism is indicated if any of the radial lines appear distorted and blurred. Record your results in the worksheet. Lab Activity 6 Test for Astigmatism Test for the Optic Disc (blind spot) The optic disc (blind spot) is formed by the convergence of the fibers (axons) of the ganglion cells of the retina. The axons pierce the posterior wall of the eye and exit to form the optic nerve. The optic disc (blind spot) contains no photoreceptors and is demonstrated by the failure of the retina to produce an image at its location. Figure An astigmatism test chart of radial lines. An astigmatism produces distortion and blurring of the radial lines. The numbers are used to identify the plane of the astigmatism. 26

27 Lab Activity 7 Test for the Optic Disc (blind spot) The optic disc (blind spot) is formed by the convergence of the fibers (axons) of the ganglion cells of the retina. The axons pierce the posterior wall of the eye and exit to form the optic nerve. The optic disc (blind spot) contains no photoreceptors and is demonstrated by the failure of the retina to produce an image at its location. Figure The optic disc (blind spot) is formed by the convergence of the fibers (axons) of the ganglion cells of the retina. Lab Activity 7 Test for the Optic Disc (blind spot) Procedure The blind spot (optic disc) is easy to demonstrate by observing two near objects points printed on paper, the eye is focused on one object and the other object is placed in the lateral visual field. The object in the lateral visual field is the point used to demonstrate the blind spot because it is focused by the lens to the medial retinal surface (see the above illustration) where the blind spot (optic disc) is located. As the image is moved away (or toward) the eye, the image in the lateral visual field is moved across the medial retinal surface. When the image moves across the blind spot (optic disc) the image disappears because the blind spot (optic disc) lacks the retina. Figure A dot and square can be used to demonstrate the blind spot (optic disc). Test for the Optic Disc (blind spot) Test for the Right Eye: 1. Close your left eye 2. Hold this page a couple of inches from your right eye and observe the above illustration of the dot and square. 3. Position the dot directly in front of the right eye. Looking directly at the dot, the square will be observed in your peripheral vision. 4. While keeping the dot sharply in focus, move the page slowly away from your right eye. 5. When the page is about 5-7 inches from the eye, the square, seen in peripheral vision, should completely disappear from your visual field. Test for the Optic Disc (blind spot) Test for the Left Eye: 1. Close your right eye 2. Hold this page a couple of inches from your left eye and observe the above illustration of the dot and square. 3. Position the square directly in front of the left eye. Looking directly at the square, the dot will be observed in your peripheral vision. 4. While keeping the dot sharply in focus, move the page slowly away from your right eye. 5. When the page is about 5-7 inches from the eye, the dot, seen in peripheral vision, should completely disappear from your visual field. 27

28 Lab Activity 7 Test for the Near Point of Vision The normal lens of the eye is extremely flexible. It can be stretched, becoming flatter and less curved, or it can recoil and become thicker and more curved. When the ciliary muscle contracts, tension on the lens is reduced. The lens recoils and becomes thicker and more curved. Increasing convexity of the lens allows focusing on near objects. When maximum convexity the lens is reached the lens is adjusted for the near point of focus. The near point of focus is the shortest distance at which the eye can sharply form a focused image. Lab Activity 7 Test for the Near Point of Vision With aging, the lens gradually loses its elasticity and loses its ability to recoil. The loss of elasticity results in the lens remaining flatter and the near point advancing away from the eye. The loss of lens elasticity with advancing near point of focus results in presbyopia, farsightedness due to advancing age. Lab Activity 7 Test for the Near Point of Vision The following illustration is an overview showing an increased distance for the near point of focus with increasing age, predicted near point. The ten year span between the ages of 30 and 40 shows the most dramatic increases in the changes in the near point of focus. Figure An overview of the changes in the near point of focus with advancing age. Lab Activity 7 Test for the Near Point of Vision Left eye Procedure 1. Close your right eye. 2. Hold this page at arm s length and focus on this letter, R. 3. While focusing on the letter R bring the page toward your eye until the letter R just becomes blurred. 4. Measure this distance in inches and record the measurement in the worksheet. Lab Activity 7 Test for the Near Point of Vision Right eye Procedure 1. Close your left eye. 2. Hold this page at arm s length and again focus on the letter R. 3. Bring the page toward your eye until the letter R just becomes blurred. 4. Measure this distance in inches and record the measurement in the worksheet. 28