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Questions and Answers
What is the average diameter of the cornea at full term birth?
At what age does the corneal power reduce to approximately 44 D?
What is the primary change in the anterior chamber depth from birth to adolescence?
What happens to the lens's dioptric power during the first years of life?
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What indicates a potential problem associated with a large diameter cornea?
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What is the primary function of aqueous humor in the anterior segment of the eye?
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Which structures are included in the anterior segment of the eye?
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What is the primary role of the cornea in the eye?
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Which layer of the cornea is responsible for regulating fluid and solute transport?
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How does the crystalline lens assist in vision?
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What role do zonular fibers play in relation to the lens?
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Why do the cells of the corneal endothelium not regenerate?
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What contributes to the transparency of the cornea?
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Which component forms the outermost layer of the crystalline lens?
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What is the approximate power of the crystalline lens?
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What is the function of the conjunctiva?
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What thickness range does the cornea have centrally?
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What role do immunoglobulins in aqueous humor play?
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What type of tissue makes up the corneal epithelium?
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What is the general shape of the ciliary body?
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What is the diameter of the cornea?
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What is the primary function of the conjunctiva in the eye?
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Which statement about the sclera is incorrect?
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What are the two types of muscles found in the iris responsible for controlling pupil size?
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What is heterochromia in relation to the eye?
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How is the pupil affected by alcohol and opioids?
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What is the role of the aqueous humor in the eye?
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Which layer is NOT part of the iris structure?
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What causes the pupil to become narrower in response to light?
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What is the primary role of the ciliary muscle in relation to the lens?
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What happens to the lens when the ciliary muscle relaxes?
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What is the significance of a narrow iridocorneal angle?
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Which of the following statements about the vitreous is correct?
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Which type of photoreceptor is primarily responsible for color vision?
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What is the outermost layer of the retina?
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Which layer of the retina contains the nuclei of ganglion cells?
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How much of the vitreous volume is composed of water?
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Study Notes
Cornea
- Diameter: 11.5 mm
- Thickness: 0.5–0.6 mm centrally and 0.6–0.8 mm peripherally.
- Transparent front part of the eye.
- Covers the iris, pupil, and anterior chamber.
- Refracts light, accounting for approximately two-thirds of the eye's total optical power (40 D).
- Very sensitive.
- No blood vessels.
- Receives nutrients via diffusion from the tear fluid at the outside and the aqueous humor at the inside.
Corneal Layers
- Corneal epithelium: thin epithelial multicellular tissue layer of fast-growing and easily-regenerated cells, kept moist with tears.
- Bowman's layer: tough layer that protects the corneal stroma.
- Corneal stroma: thick, transparent middle layer, consisting of regularly- arranged collagen fibers along with sparsely distributed interconnected keratocytes, which are the cells for general repair and maintenance.
- Descemet's membrane: thin acellular layer that serves as the modified basement membrane of the corneal endothelium, from which the cells are derived.
- Corneal endothelium: simple squamous or low cuboidal monolayer of mitochondria-rich cells responsible for regulating fluid and solute transport between the aqueous and corneal stromal compartments. This layer is bathed by aqueous humor. Unlike the corneal epithelium, these cells do not regenerate. Instead, they stretch to compensate for dead cells. If the endothelium can no longer maintain a proper fluid balance, stromal swelling due to excess fluids and subsequent loss of transparency will occur.
Transparency of the Cornea
- Two theories explain how transparency in the cornea comes about:
- Lattice arrangements of the collagen fibrils in the stroma: Light scatter by individual fibrils is cancelled by destructive interference from the scattered light from other individual fibrils.
- Spacing of the neighboring collagen fibrils in the stroma: The spacing must be < 200 nm for there to be transparency.
Conjunctiva
- Clear mucous membrane consisting of cells and underlying basement membrane that covers the sclera (white part of the eye) and lines the inside of the eyelids.
- Helps lubricate the eye by producing mucus and tears.
- Contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.
Sclera
- White part of the eye.
- Opaque due to the irregularity of the collagen fibers.
- Vascular.
- In children, it is thinner and shows some of the underlying pigment, appearing slightly blue.
- In the elderly, fatty deposits on the sclera can make it appear slightly yellow.
Iris
- The colored part of the eye, the other structures visible are the pupil in the center and the white sclera surrounding the iris.
- Controls the diameter and size of the pupil and the amount of light reaching the retina.
- Consists of two layers:
- Stroma: front pigmented fibrovascular tissue
- Pigmented epithelial cells: beneath the stroma.
- Has two muscles:
- Sphincter: responsible for pupillary constriction.
- Dilator: responsible for pupillary dilation.
- Usually strongly pigmented, with colors ranging from brown to green, blue, grey, and hazel.
- Occasionally light color is due to lack of pigmentation (melanin).
- Heterochromia: ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). Uncommon in humans but can indicate ocular disease.
Pupil
- An opening at the center of the iris, NOT A STRUCTURE.
- The dilator pupillae, innervated by sympathetic nerves.
- The pupil is constricted by the parasympathetic nerves.
- Atropine, cocaine, and amphetamines drug (cycloplegia) mimics the action of the sympathetic nerves.
- Alcohol and opioids mimics the action of the parasympathetic nerves.
- Gets wider in the dark but narrower in light.
Aqueous Humor
- Thick watery substance filling the space between the lens and the cornea.
- Secreted into the posterior chamber by the ciliary body.
- Anterior segment: front third of the eye that includes the structures in front of the vitreous humor: the cornea, iris, ciliary body, and lens.
- Anterior chamber: Within the anterior segment, the space between the posterior surface of the cornea (corneal endothelium) and the iris.
- Posterior chamber: Within the anterior segment, the space between the iris and the front face of the vitreous.
Functions of Aqueous Humor
- Provides diopteric power to the cornea.
- Maintains the intraocular pressure and inflates the globe of the eye.
- Provides nutrition (e.g.amino acids and glucose) for the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous.
- Carries away waste products from metabolism of the above avascular ocular tissues.
- May serve to transport ascorbate in the anterior segment to act as an anti- oxidant agent.
- Presence of immunoglobulins indicate a role in immune response to defend against pathogens.
- Maintains proper brain energy consumption.
Crystalline Lens and Zonules
- Transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina.
- Its power is approximately 20 D.
- Changes shape to change the focal distance of the eye so that it can focus on objects at various distances (accommodation).
- Lens is suspended in place by the zonular fibers, which attach to the lens near its equatorial line and connect the lens to the ciliary body.
- Consists of three main parts:
- Lens capsule: outermost layer of the lens.
- Lens epithelium: inner layer of the lens.
- Lens fibers: bulk of the interior of the lens.
Ciliary Body
- Circumferential tissue inside the eye composed of the ciliary muscle and ciliary processes.
- Triangular in horizontal section, and is coated by a double layer, the ciliary epithelium.
- Extends from the ora serrata to the root of the iris.
- Attached to the lens by connective tissue called the zonule of Zinn.
- Responsible for shaping the lens to focus light on the retina.
- When the ciliary muscle contracts, the lens becomes more convex, generally improving the focus for closer objects.
- When it relaxes, it flattens the lens, generally improving the focus for farther objects.
Functions of the Ciliary Body
- Accommodation
- Aqueous humor production
- Production and maintenance of the lens zonules.
Irido corneal angle
- The width of the iridocorneal angle is one factor affecting the drainage of aqueous humor from the eye's anterior chamber.
- A wide angle allows sufficient drainage of humor through the trabecular meshwork.
- A narrow angle may impede the drainage system and leave the patient susceptible to acute angle-closure glaucoma.
Vitreous
- Transparent, colourless, gelatinous mass that fills the space between the lens of the eye and the retina lining the back of the eye.
- Produced by certain retinal cells
- Has no blood vessels.
- 98-99% of its volume is water.
- Helps to keep the retina in place.
- It adheres to the retina in three places only: around the anterior border of the retina; in the macula; and at the optic nerve disc.
Retina
- Complex, layered structure with several layers of neurons interconnected by synapses.
- The only neurons that are directly sensitive to light are the photoreceptor cells.
- There are two types of photoreceptor cells:
- Rods: function mainly in dim light and provide black-and-white vision.
- Cones: support daytime vision and the perception of colour.
Layers of the Retina
- Inner limiting membrane: Müller cell footplates
- Nerve fiber layer
- Ganglion cell layer: Layer that contains nuclei of ganglion cells and gives rise to optic nerve fibers.
- Inner plexiform layer
- Inner nuclear layer: contains bipolar cells.
- Outer plexiform layer: In the macular region, this is known as the Fiber layer of Henle.
- Outer nuclear layer
- External limiting membrane: Layer that separates the inner segment portions of the photoreceptors from their cell nuclei.
- Photoreceptor layer: Rods / Cones
- Retinal pigment epithelium
Developmental Aspects
-
Corneal power:
- 60 D at 28 weeks of gestational age.
- 54 D at 30-35 weeks.
- 48,4 D- 51 D at birth.
- 44 D at 1 year of life.
- Flattening in the late teens.
-
Anterior chamber depth:
- At birth, it averages 2,6 mm.
- It deepens by about 1,4 mm from birth to adolescence.
- Most of this change happens in the first 18 months.
- Lens: - 43,3 D in preterm neonates. - 34,4 D in full term neonates (almost spherical). - During the first years of life, there is an increase in the transverse diameter and flattening of the lens surfaces. - Thus the lens becomes less spherical and its dioptric power decreases. - This, together with the flattening cornea, compensates for the increasing axial length. - Changes in the lens continue up to the middle teenage years. - The emmetropization occurs as coordinated growth between lens power and axial length. - If emmetropization did not occur, there would be large fluctuations in the refractive errors.
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Description
Explore the anatomy and functions of the cornea in this quiz. Learn about its dimensions, transparency, and the different layers that comprise this crucial part of the eye. Test your knowledge on how the cornea contributes to vision and its unique characteristics.
