Eye Anatomy And Function Notes

Special Senses

Special senses or special sensations are complex sensations for which the body has some specialized sense organs. These sensations are different from somatic sensations that arise from skin, muscles, tendons, and joints.

Special senses are:

• Sensation of vision
• Sensation of hearing
• Sensation of taste
• The sensation of smell.

Read And Learn More: Medical Physiology Notes

Functional Anatomy Of The Eyeball

1. Morphology:

• The human eyeball (bulbus oculi) is approximately globe-shaped with a diameter of about 24 mm. It is slightly flattened from above downwards. The eyeball is made up of two segments, an anterior part, and a posterior part.
• The anterior part is small and forms 1/6 of the eyeball. The posterior part is larger and forms 5/6 of the eyeball. The radius of this part is about 8 mm. The posterior wall of this part is lined by the light-sensitive structure called retina.
• The center of the anterior curvature of the eyeball is called the anterior pole, and the center of the posterior curvature is called the posterior pole. The line joining the two poles is called the optic axis. The line is joining a point IP.
• cornea little medial to the anterior pole and the fovea centralis situated lateral to the posterior pole is known as the visual axis. The light rays pass through the visual axis of the eyeball. The optic nerve leaves the eye, a little medial to the posterior pole.

2. Orbital Cavity:

• Except for the anterior 1/6, the eyeball is situated in the bony cavity known as the orbital cavity or eye socket. A thick layer of areolar tissue is interposed between the bone and the eye.
• It serves as a cushion to protect the eyeball from external force. The eyeballs are attached to the orbital cavity by the ocular muscles.

3. Eyelids:

• Eyelids protect the eyeball from foreign particles coming in contact with its surface and cut off the light during sleep. The eyelids are opened and closed voluntarily as well as reflexly.
• The margins of eyelids have sensitive hairs called cilia. Each cilium arises from a follicle, which is surrounded by a sensory nerve plexus. When the dust particle comes in contact with cilia, these sensory nerves are activated resulting in rapid blinking of eyelids. It prevents the dust particles from reaching (the eyeball.
• There are about 100-150 cilia in the upper eyelid and about 50-75 cilia in the lower eyelid. Meibomian glands and some sebaceous glands are also found in the eyelids. These glands open into the follicles of cilia. The infection of these glands leads to the development of common eye sty.
• The opening between the two eyelids is called palpebral fissure. In adults, it is about 25 mm long. Its width is about 12-15 mm when opened.

4. Conjunctiva:

• It is a thin mucous membrane, which covers the exposed part of the eye. After covering the anterior surface, the conjunctiva is reflected into the inner surfaces of the eyelids. The part of the conjunctiva covering the eyeball is called the bulbar portion. The part covering the eyelid is called the palpebral portion.
• When the eyelids are closed or opened, the opposed portions of conjunctiva slide over each other. The surface of the conjunctiva is lubricated by a thin film of tears secreted by the lacrimal gland.

5. Lacrimal Glased:

• The lacrimal gland is situated in the shelter of bone, forming the upper and outer border of the wall of the eye socket. From the lacrimal gland, a tear flows over the surface of the conjunctiva and drains into the nose via lacrimal ducts, lacrimal sac, and nasolacrimal duct.
• The tear is a hypertonic fluid. Due to its continuous washing and lubrication, the conjunctiva is kept moist and is protected from infection. Tear also contains lysozyme that kills bacteria. Secretion of tears is controlled by the parasympathetic fibers of the facial (VII cranial) nerve.

Wall Of The Eyeball

The wall of the eyeball is composed of three layers:

1. The outer layer, which includes the cornea and sclera
2. The middle layer, which includes the choroid, ciliary body, and iris
3. The inner layer is the retina.

1. Outer Layer Or Tunica Externa Or Tunica Fibrosa: The outer layer preserves the shape of the eyeball. The posterior 5/6 of this coat is opaque and it is called the sclera. The anterior 1/6 is transparent and is known as the cornea.

1. Sclera:

• Sclera is the tough white fibrous outer layer of the eyeball that covers the posterior 5/6 of the eye. Anteriorly it is continuous with the cornea.
• Sciera is formed by white fibrous tissues and elastic fibers. The posterior part of the sclera where it is pierced by the optic nerve is thin with perforations. It is named as lamina cribrosa.

2. Cornea:

• The cornea is the transparent convex anterior portion of the outer layer of the eyeball which covers the iris and the pupil. It forms the anterior 1/6 of the outer layer and it is continuous with the sclera.
• Though the cornea is transparent, it does not appear transparent. It appears in different colors such as blue, green, brown, gray, and black. It is because of the color of the iris which is present just behind the cornea. The sclera overlaps the cornea at its periphery and appears in front as the white of the eye. The diameter of the cornea is about 12 mm horizontally and 11 mm vertically.

The cornea is formed by five layers:

1. A layer of stratified epithelium
2. Bowman’s membrane or anterior elastic lamina
3. Substantial proper
4. Descemet’s layer or posterior elastic lamina
5. A layer of endothelial cells.

• The cornea has a refractory index of 1.376. It is very sensitive to pain, touch, pressure, and cold. The center of the cornea is more sensitive to pain because of a rich supply of free nerve endings. Normally, the cornea is not vascularized. Therefore, it derives its nourishment mainly from aqueous humor. However, in some pathological conditions, the cornea becomes vascularized.
• The transitional part of the outer layer between the sclera and cornea is called the limbus. It is about 1 mm in width. Only at the limbus, the blood vessels are seen which form a superficial marginal plexus in the limbus.

2. Middle Layer Or Tunica Media Or Tunica Vasculosa: The middle layer surrounds the eyeball completely except for a small opening in front known as the pupil. This layer comprises three structures.

1. Choroid
2. Ciliary body
3. Iris.

1. Choroid:

• The choroid is the thin vascular layer of the eyeball situated between the sclera and retina. It forms posterior 5/6 of the middle layer. The choroid is extended anteriorly up to the insertion of the ciliary muscle (the level of ora serrata). The choroid is separated from the sclera by perichoroidal space. Anteriorly, this space is limited by the insertion of ciliary muscle into the sclera.
• Posteriorly, this space ends at a short distance from the optic nerve. The inner surface of the choroid faces the pigment epithelium (innermost layer) of the retina. The choroid is composed of a rich capillary plexus, numerous small arteries, and veins.

2. Ciliary Body:

• The ciliary body is the thickened anterior part of the middle layer of the eye situated between the choroid and iris. It is situated in front of the ora serrata.
• It is in the form of a ring. Its outer surface is separated from the sclera by perichoroidal space. The inner surface of the ciliary body faces the vitreous body and lens. The suspensory ligaments from the lens are attached to the ciliary body. The anterior surface of the ciliary body faces towards the center of the cornea. From the surface, the iris arises.

The ciliary body has three parts:

1. Orbiculus ciliaris
2. Ciliary body proper
3. Ciliary processes.

 

1. Orbiculus ciliaris: It is continuous with choroid and it forms the posterior 2/3 of the ciliary body. It is about 4 mm broad.
2. Ciliary body proper: It is made up of two sets of ciliary muscles namely, the outer longitudinal and inner circular muscles. The ciliary muscles are innervated by the parasympathetic fibers of
   oculomotor nerve.
3. The ciliary processes: The ciliary processes are the finger-like projections from the inner surface si the ciliary body. There are about 70 ciliary processes, projecting towards the central axis of the eye to form radial fringes called corona ciliaris.

3. Iris: Iris is the thin-colored curtain-like structure of the eyeball. It forms the anterior part of the middle layer. It is like a thin circular diaphragm, placed in front of the lens. It has a circular opening in the center called the pupil.

• Iris is a muscular structure and has two muscles:

1. Constrictor pupillae
2. Dilator pupillae.

1. Constrictor pupillae: It is also called iris sphincter muscle or sphincter pupillae. It is formed by circular muscle fibers. Con¬traction of this muscle causes constriction of the pupil.

2. Dilator pupillae: It is also called pupillary dilator muscle. It is formed by radial muscle fibers. Contraction of this muscle causes dilatation of the pupil.

• The activities of these muscles of the iris increase or decrease the diameter of the pupil and regulate the amount of light entering the eye. Thus, the iris acts like the diaphragm of a camera.
• Iris separates the space between the cornea and lens into two chambers namely, the anterior and posterior chambers. Both chambers communicate with each other through the pupil. The lateral border of the anterior chamber is angular in shape. It is called the iris angle or the angle of the anterior chamber.

3. Inner Layer Or Tunica Interna Or Tunica Nervosa Or Retina:

The retina is a delicate light-sensitive membrane that forms the innermost layer of the eyeball. It extends from the margin of the optic disk to just behind the ciliary body. Here, it ends abruptly as a dentated border known as ora serrata. The retina has the receptors of vision. Structurally, the retina is made up of 10 layers.

The layers of the retina from the outside are:

1. A layer of pigment epithelium
2. A layer of rods and cones
3. External limiting membrane
4. Outer nuclear layer
5. Outer plexiform layer
6. Inner nuclear layer
7. Inner plexiform layer
8. Ganglion cell layer
9. A layer of nerve fibers
10. Internal limiting membrane.

1. Layer of Pigment Epithelium:

• It is the outermost layer situated adjacent to the choroid. It is a single layer of hexagonal epithelial cells. The outer portion of the epithelial cells (towards the choroid), contains the nucleus and a moderate number of round pigment granules.
• The inner portion has plenty of needle-shaped dark pigment granules. Many proto¬plasmic extensions arise from the inner surface of cells and pass between the rods and cones.
• The cytoplasmic processes also contain dark pigment granules. The pigment present in this layer is a melanin called fuscin. The pigment epithelial layer absorbs light and prevents the reflection of light rays back from the retina.
• If light rays are reflected back by the retina, the image becomes blurred. The epithelial cells store vitamin A (retinol) and remove the debris from rod cells and cone cells by phagocytic action.

2. Layer of Rods and Cones:

• This layer lies between the pigment epithelial layer and the external limiting membrane. The rods and cones are the light-sensitive portions of the visual receptor cells, the rod cells, and the cone cells.
• The receptor cells are arranged in a parallel fashion and are perpendicular to the inner surface of the eyeball. The structure of rod cells and cone cells is explained in the next chapter.

3. External Limiting Membrane: It is a thin layer, formed by the chief supporting elements of the retina called the Muller’s fibers.

4. Outer Nuclear Layer: The fibers and granules of rods and cones are present in this layer. The granules of rods and cones contain a nucleus. The nuclei of rods are smaller and round and, the nuclei of cones are larger and oval in shape.

5. Outer Plexiform Layer: This layer contains reticular meshwork formed by the terminal fibers of rods and cones and the dendrites from bipolar cells, situated in the inner nuclear layer.

6. Inner Nuclear Layer:

• The inner nuclear layer contains small oval-shaped flattened bipolar cells. The axons of the bipolar cells go inside and synapse with the dendrites of ganglionic cells in the inner plexiform layer.
• The dendrites synapse with fibers of rods and cones in the outer plexiform layer. This layer also contains nuclei of Muller’s supporting fibers and some association neurons. The association neurons are horizontal cells and amacrine cells.

7. Inner Plexiform Layer: This layer of the retina consists of synapses between the dendrites of ganglionic cells and axons of bipolar cells. It also contains processes from amacrine cells.

8. Ganglion Cell Layer:

• Multipolar cells are present in this layer. Some cells are large and are called giant ganglion cells. Other cells are smaller called midget ganglion cells. The axons from ganglion cells are on the inner surface of the retina.
• These axons form the optic nerve. The dendrites of the ganglion cells synapse with axons of bipolar cells in the inner plexiform layer. This layer also contains retinal blood vessels.

9. Layer of Nerve Fibers:

• It is formed by nonmyelinated axons of ganglionic cells. After taking the origin, the axons run horizontally to a short distance. Afterward, the fibers converge towards the optic disk and form the optic nerve.
• This layer also consists of neuroglial cells, Muller’s cells, and retinal blood vessels.

10. Internal Limiting Membrane: It is the innermost layer of the retina and it separates the retina from the vitreous body. It is a hyaline membrane formed by the opposition of expanded ends of Muller’s fibers.

Fundus Oculi

Fundus oculi is the posterior part of the interior of the eyeball. It is also called the fundus. In living subjects, the fundus is examined by an ophthalmoscope.

The fundus has two important structures:

1. Optic disk
2. Macula (tea with fovea centralis.

1. Optic Disk – Blind Spot:

• The optic disk is a pale disk situated near the center of the posterior wall of the eyeball. It is also called optic papilla. It is formed by the convergence of axons from ganglion cells while forming the optic nerve. The optic disk contains all the layers of the retina except rods and cones.
• Therefore, it is insensitive to light, i.e. the object is not seen if its image falls upon this area. Because of this, the optic disk is known as a blind spot.

2. Macula Lutea: Macula lutea is a small yellowish area situated a little lateral to the optic disk of the retina. It is also called a yellow spot. The yellow color of the macula lutea is due to the presence of a yellow pigment. Macula lutea has a fovea centralis in its center.

• Fovea Centralis:
  • Fovea centralis is a minute depression in the center of the macula lutea. Here, all the layers of the retina are very thin. The diameter of the fovea is only about 0.5 mm. The fovea is the region of the most acute vision because it contains only the cones.
  • When one looks at an object, the eyeballs are directed towards the object, so that, the image of that object falls on the fovea of each eye and the person can see the object very clearly. It is known as foveal vision.
  • The vision in the rest of the retina is called peripheral or extrafoveal vision. It is less sensitive and enables the subject to gain only a dim and ill-defined impression of surroundings.
  • Degeneration of macula lutea is a common cause of blindness.

3. Intraocular Fluid: The fluid in the eyeball is responsible for the maintenance of the shape of the eyeball. Two types of fluids are present in the eye:

1. Vitreous humor
2. Aqueous humor.

1. Vitreous Humor:

• Vitreous humor is a viscous fluid present behind the lens in the space between the lens and the retina. It is a highly viscous and gelatinous substance. It is also known as a vitreous body. It is formed by a fine fibrillar network of proteoglycan molecules.
• The major substances in vitreous humor are albumin and hyaluronic acid. These substances enter the vitreous body from the blood by means of diffusion. Vitreous humor helps maintain the shape of the eyeball.

2. Aqueous Humor: Aqueous humor is a thin fluid present in front of the retina. It fills the space between the lens and the cornea. This space is divided into anterior and posterior chambers by the iris. Both chambers communicate with each other through the pupil.

Properties of Aqueous Humor:

Volume:  0.13 mL

Reaction and pH:  Alkaline with a pH of 7.5

Viscosity:  1.029

Refractory s’ and ex:  1.34

Composition of Aqueous Humor: Composition of aqueous humor is given in.

Formation of Aqueous Humor:

• Aqueous humor is formed by ciliary processes. It is formed from plasma within the capillary network of the ciliary process by diffusion, ultrafiltration, and active transport through the epithelial cells lining the ciliary processes.
• After formation, aqueous humor reaches the posterior chamber bypassing through the suspensory ligaments. From here it reaches the anterior chamber via the pupil. The formation of aqueous humor is a continuous process.
• The rate of formation is about 2-3 pL per minute. The amount of aqueous humor in the anterior chamber is about 230-250 pL and in the posterior chamber, it is about 50-60 pL.

Drainage of Aqueous Humor: From the anterior chamber, the aqueous humor passes into the angle between the cornea and iris called the limbus. From here, the fluid passes through a meshwork of trabeculae situated near the junction of the iris and cornea. Then it flows through the canal of Schlemm and reaches the venous system via an anterior ciliary vein.

Functions of Aqueous Humor Aqueous humor:

• Maintains the shape of the eyeball
• Maintains the intraocular pressure
• Provides nutrients, oxygen, and electrolytes to the avascular structures like lens and cornea
• Removes metabolic end products from lens and cornea

Intraocular Pressure

• Intraocular pressure is the measure of fluid pressure in the eye exerted by aqueous humor. The normal intraocular pressure varies between 12 and 20 mm Hg.
• Measurement of intraocular pressure is an important part of eye examination. It is measured by a tonometer. When intraocular pressure increases to about 60-70 mm Hg, glaucoma occurs. Refer to Applied Physiology in this chapter for details.

Lens

• The lens of the eyeball is crystalline in nature. It is situated behind the pupil. It is a biconvex, transparent, and elastic structure. It is avascular and receives its nutrition mainly from the aqueous humor.
• The lens refracts light rays and helps to focus the image of the objects on the retina. The focal length of the human lens is 44 mm and its refractory power is 23D. Lens is supported by the suspensory ligaments (zonular fibers) which are attached to ciliary bodies.

1. Structure Of The Lens: The lens is formed of three components:

1. The capsule
2. The anterior epithelium
3. The lens substance.

1. The Capsule: Capsule is a highly elastic membrane that covers the lens.

2. The Anterior Epithelium: It is a single layer of cuboidal epithelial cells situated beneath the capsule. At the margins, the epithelial cells are elongated. The epithelial cells give rise to the lens fibers present in the lens substance.

3. The Lens Substance: The lens is formed by long lens fibers derived from the anterior epithelium. The lens fibers are prismatic in nature and are arranged in concentric layers.

2. Changes In The Lens During Old Age:

• Like any other tissue in old age, the elastic property of tens also is decreased. The decreased elasticity of the lens is due to the physical changes in the lens and its capsule and It causes presbyopia.
• In old age, the lens becomes opaque and this condition is called cataract. Refer to Applied Physiology in this chapter for details.

Ocular Muscles

1. Muscles Of The Eyeball: The muscles of the eyeball are of two types:

1. Intrinsic muscles
2. Extrinsic muscles.

1. Intrinsic Muscles:

• The intrinsic muscles are formed by smooth muscle fibers and are controlled by the autonomic nerves. The intrinsic muscles of the eye are constrictor pupillae, dilator pupillae, and ciliary muscle.
• The actions of constrictor pupillae and dilator pupillae are already explained along with the iris. Contraction of ciliary muscle increases the anterior curvature of the lens during accommodation.

2. Extrinsic Muscles:

• Generally, the term ocular muscles refers to the extrinsic muscles of the eyeball. The extrinsic muscles are formed by skeletal muscle fibers and are controlled by the somatic nerves. The eyeball moves within the orbit by six extrinsic skeletal muscles.
• One end of each muscle is attached to the eyeball and the other end to the wall of the orbital cavity. There are four straight muscles (rectus) and two oblique muscles:
  • Superior rectus
  • Inferior rectus
  • Medial or internal rectus
  • Lateral or external rectus
  • Superior oblique
  • Inferior oblique.

2. Innervation Of Ocular Muscles:

• Innervation of intrinsic Muscles: The intrinsic muscles of the eyeball are innervated by both sympathetic and parasympathetic divisions of the auto¬nomic nervous system.
  • Parasympathetic nerve fibers:
    • The parasympathetic preganglionic fibers arise from the Edinger-Westphal nucleus of 3 cranial nerves. After passing through 3 cranial nerves, these fibers synapse with postganglionic neurons in the ciliary ganglion.
    • The postganglionic fibers arising from here pass through ciliary nerves and innervate the ciliary muscle and constrictor pupillae. Stimulation of parasympathetic nerve fibers causes contraction of the ciliary muscle and constrictor pupillae.
  • Sympathetic nerve fibers:
    • The sympathetic preganglionic nerve fibers arise from the lateral horn of the first thoracic segment of spinal CCR-‘T VBGJ; through the sympathetic chain and synapse with of superior cervical sympathetic ganglion.
    • Postganglionic fibers arising from this ganglion run along the carotid artery and its branches to reach the intrinsic muscles of the eyeball. Stimulation of sympathetic nerve fibers causes relaxation of ciliary muscle and contraction of dilator pupillae.
• Innervation of Extrinsic Muscles:

The extrinsic muscles of the eyeball are innervated by somatic motor nerve fibers. The somatic nerve fibers arise from the cranial nerve nuclei in the brainstem and reach the ocular muscles via three cranial nerves:

1. Oculomotor (third) nerve
2. Trochlear (fourth) nerve
3. The abducent (sixth) nerve.

1. Oculomotor Nerve: The oculomotor nerve supplies:

• Superior rectus
• Inferior rectus.
• Medial rectus (internal rectus)
• Inferior oblique.

2. Trochlear Nerve: The trochlear nerve supplies the superior oblique.

3. Abducent Nerve: The abducent nerve supplies the lateral rectus (external rectus).

Ocular Movements

The eyeball moves or rotates within the orbital socket in any of the three primary axes, vertical, transverse, and anteroposterior axis.

1. Movements In Vertical Axis: Movements of the eyeball in the vertical axis or in the horizontal plane are of two types.

1. Abduction or Lateral Movement or Outward Movement: The abduction of the eyeball is due to the contraction of the lateral rectus mainly. It is supported by the two oblique muscles.
2. Adduction or Medial Movement or Inward Movement: The adduction of the eyeball occurs because of the action of the medial or internal rectus along with the action of the superior rectus and inferior rectus.

2. Movements In Transverse Axis: Following are the movements of eyeballs in the transverse axis or in the sagittal plane.

• Elevation or Upward Movement: Elevation of the eyeball occurs because of the cont¬raction of the superior rectus and the inferior oblique muscles.
• Depression or Downward Movement: Depression of the eyeball is brought out by the inferior rectus and superior oblique.

3. Movements In Anteroposterior Axis: The movements of the eyeball in the anteroposterior axis or in use frontal plane are called torsion or wheel movements. The two types of torsion movements are extorsion and intorsion.

1. Extorsion: During extorsion, the eyeball is rotated in such a way that the cornea turns upward and outward direction. This movement is due to the contraction of the inferior oblique and inferior rectus.
2. Intorsion: During intorsion, the eyeball is rotated so that, the cornea moves in a downward and inward direction. It is produced by the contraction of the superior oblique and superior rectus muscles.

4. Simultaneous Movements Of Both Eyeballs: Both eyeballs show many types of simultaneous movements, namely conjugate movements, disjugate movements, pursuit movements,s, and saccadic movements.

1. Conjugate Movement: Conjugate movement is the movement of both eyeballs in the same direction. The visual axes of both eyes remain parallel. It is due to the contraction of the medial rectus of one eye and the lateral rectus of the other eye.

2. Disjugate Movement: Disjugate movement is the movement of both eyeballs in opposite directions. There are two types of disjugate movement namely, convergence and divergence.

• Convergence: Convergence is the movement of both eyeballs towards the nose. It is due to the simultaneous contraction of the medial rectus and simultaneous relaxation of the lateral rectus of both eyes. The visual axes move close to each other. Convergence of eyeballs occurs during accommodation.
• Divergence: Divergence is the movement of both eyeballs towards the temporal side. It is due to the simultaneous contraction of the lateral rectus and simultaneous relaxation of the medial rectus of both eyes. The visual axes of the eyes move away from each other.

3. Persuit Movement: Persuit movement is the movement of eyeballs along with an object when eyeballs are fixed on a moving object.

4. Saccadic Movement: Saccadic movement is the quick jerky movement of both eyeballs when the fixation of eyes (gaze) is shifted from one object to another object. It is also called optokinetic movement.