anatomy

Did you get it?
1. What’s the role of eyelids?
To protect the eyes.
2. Which structure of eye forms tears?
Lacrimal glands
3. What are tears?
A dilute saline solution containing lysozyme and antibodies.
4. What’s the visual role of the external eye muscles?
They direct the eyeball toward what you wish to see.
5. What is the meaning of the term blind spot in relation to the eye?
The blind spot contains no photoreceptors; it is the site where the optic nerve leaves the eyeball
6. What function does the choroid of the vascular layer have in common with the pigmented layer of the retina?
Both contain pigment, which prevents light scattering in the eye.
7. How do the rods and cones differ from each other?
Rods have a rod-like outer segment containing the photopigment.
Cones have a shorter cone-shaped outer segment.

Rods respond to low light conditions and produce black and white vision.
Cones need bright light and provide colour vision.

8. What are the refractory media of the eye?
Refractory media include the cornea, aqueous humor, lens, vitreous humor.
9. What name is given to the ability of the eye to focus on close objects? accommodation 10. What is the difference between the optic tract and the optic nerve?
The optic nerves leave the eyeballs, and the medial half of the fibers of each optic nerve cross over to the opposite side, joining there with the fibers from the outside half of the opposite eye to form the optic tracts.
11. In what way does the photopupillary reflex protect the eyes?
It causes pupillary constriction in very bright light. Intense light stimulation can injure the photoreceptors.
12. Which layer of the eye would be the first to be affected by deficient tear production?
The outermost fibrous layer (sclera and especially its cornea), which normally is continuously washed by tears

Summary
1. External/ accessory structures of the eye:
a. Extrinsic eye muscles aim the eyes for following moving objects and for convergence.
b. The lacrimal apparatus includes a series of ducts and the lacrimal glands that produce a saline solution, which washes and lubricates the eyeball.
c. Eyelids protect the eyes. Associated with the eyelashes are the ciliary glands (modified sweat glands) and the tarsal glands (which produce an oily secretion that helps keep the eyes lubricated).
d. The conjunctiva is a mucous membrane that covers the anterior eyeball and lines the eyelids. It produces a lubricating mucus.
2. 3 layers form the eyeball
a. The sclera forms most of the outer, tough, protective fibrous layer. The anterior portion is the cornea, which is transparent to allow light to enter the eyes.
b. The vascular layer, or middle coat, provides nutrition to the internal eye structures. Its posterior portion, the choroid, is pigmented and prevents light’s scattering in the eye. Anterior modifications include 2 smooth muscle structures: the ciliary body and the iris (which controls the size of the pupil).
c. The sensory layer consists of the 2-layered retina – a pigmented epithelium and the innermost (neural) coat, which contains the photoreceptors. Rods are dim light receptors. Cones are receptors that provide for colour vision and high visual acuity. The fovea centralis, on which acute focusing occurs, contains only cones.
3. The blind spot (optic disc) is the point where the optic nerve leaves the back of the eyeball.
4. The lens is the major light-bending (refractory) structure of the eye. Its convexity is increased by the ciliary body for close focus. Anterior to the lens is the aqueous humor; posterior to the lens is the vitreous humor. Both humors reinforce the eye internally. The aqueous humor also provides nutrients to the avascular lens and cornea.
5. Errors of the refraction include myopia, hyperopia, and astigmatism. All are correctable with specially ground lenses.
6. The pathway of light through the eye is
Cornea  aqueous humor  (through pupil)  aqueous humor  lens  vitreous humor  retina
7. Overlap of the visual fields and inputs from both eyes to each optic cortex provide for depth perception.
8. The pathway of nerve impulses from the retina of the eye is
Optic nerve optic chiasma optic tract thalamus optic radiation visual cortex in occipital lobe of brain
9. Eye reflexes include the photopupillary, accommodation pupillary, and convergence.

Homeostatic imbalance
Inflammation of the conjunctiva, called conjunctivitis, results in reddened, irritated eyes. Pinkeye, its infectious form caused by bacteria or viruses, is highly contagious.
Because the nasal cavity mucosa is continuous with that of the lacrimal duct system, a cold or nasal inflammation often causes the lacrimal mucosa to become inflamed and swell. This impairs the drainage of tears from the eye surface, causing “watery” eyes.
Anything that interferes rod function hinders our ability to see at night, a condition called night blindness. Night blindness dangerously impairs the ability to drive safely at night. Its most common cause is prolonged vitamin A deficiency, which eventually results in deterioration of much of the neural retina. Vitamin A is one of the building blocks of the pigments the photoreceptor cells need to respond to light. Vitamin A supplements will restore function if taken before degenerative changes occur.
In youth, the lens is perfectly transparent and has the consistency of hardened jelly, but as we age it becomes increasingly hard and opaque. Cataracts, which result from this process, cause vision to become hazy and distorted, and they eventually cause blindness in the affected eye. Other risk factors for forming cataracts include diabetes mellitus, frequent exposure to intense sunlight, and heavy smoking. Current treatment of cataracts is either surgical removal of the lens and replacement with a lens implant or special cataract glasses. Photo of cataract. The cataract appears as a milky structure that seems to fill the pupil.
Hemianopia is the loss of the same side of the visual field of both eyes, which results from damage to the visual cortex on one side only (as occurs in some CVAs). Thus, the person would not be able to see things past the middle of his or her visual filed on either right or left side, depending on the site of the CVA.