Anatomy of the Eye



The Eyeball
A spherical globe with a diameter of 24.5mm.
The coats of the eye
 Three layers:
  1. The outer layer: inelastic coat, transparent      cornea and opaque sclera.
  2. The middle layer: vascular coat called as Uvea, it has 3 parts choroid, ciliary body and iris.
  3. The inner layer: The Retina, extends forwards to within 6 mm of the limbus.
(1)Outer layer


(i)Cornea
The transparent dome which serves as the window of the eye.
The primary (most powerful) structure focusing light entering the eye.

Cornea is composed of 5 layers, from the front to the back:
  1. Epithelium.
  2. Bowman’s (anterior limiting) membrane.
  3. Stroma (substantia propria).
  4. Descemet’s (posterior limiting) membrane.
  5. Endothelium (posterior epithelium).

How does the cornea stay transparent?
    • No blood vessels.
    • Transparent stroma with low level of fluids.
    • Endothelium cells serves as a pump that supply oxygen and remove fluids.
    • Tear film also supplies oxygen and keep corneal surface smooth and clean.
(ii)Sclera
  • The white, opaque cover of the eye.
  • Covers 80% of the eye’s outer layer.
  • Contains thick elastic collagen.
  • It provides protection.
  • Serves as an attachment for the extra-ocular muscles which move the eye.
(2)Middle layer
Uveal tract

  1. Iris
  2. Ciliary body
  3. Choroid
(1)Iris
A thin diaphragm composed mostly of connective tissue and smooth muscle fibers,lies between the cornea and the lens.

Iris anatomy
The iris is composed of 3 layers, from the front to the back:
  1. Endothelium
  2. Stroma
  3. Epithelium
Stroma muscles
Dilator - sympathetic innervation
Constrictor – parasympathetic innervation

Eye color (iris color)
Determined by the amount of pigment present in iris.
No pigment - pink iris (albino), some pigment – blue iris, increasing amounts of pigment- green>hazel> brown irides. .
The pigments: melanin (chromosome 15) and lipochrome (chromosome 19).
Heterochromia irides: when one iris has a different color than the other iris.
Pupil
Circular hole in the middle of the iris.
Acts like the shutter of a camera:
In darkness the iris dilator muscle causes the pupil to “dilate” and allowing more light to reach the retina.
In brightness, the iris sphincter muscle (which encircles the pupil) constricts, causing the pupil to “constrict” and allowing less light to reach the retina.
Constriction also occurs during accommodation -  the “near reflex.”
(2)Ciliary body

  • Pars plana – flat area continuous with the retina
  • Pars plicata – contains the ciliary processes that secretes the aqueous humor
  • Ciliary muscle runs circularly around the eye and controlles accommodation

Aqueous humor
Produced by the ciliary body.
Entering from the posterior chamber, it passes through the pupil into the anterior chamber and filtrates through the angle into the blood stream.
Serves to nourish ocular structures.

The intraocular pressure
The pressure within the eye is maintained at a steady level by continuous formation & drainage of aqueous.
Aqueous is secreted by the ciliary epithelium → posterior chamber → anterior chamber (through the pupil ) → drained through the anterior chamber angle.
The intraocular pressure, (IOP), is normally      10 – 21 mmHg; increased IOP called Glaucoma.
High IOP almost always due to an obstruction of aqueous outflow.
Anterior chamber angle
Iris-cornea junction.
Contains the trabecular meshwork which acts like a filter for the aqueous humor.
From the TM the humor drains to schlem’s canal and then to blood stream.
(3)Choroid
The posterior segment of the uvea, between the sclera and the retina.
Rich in blood supply, supplies oxygen and nutrition to the outer two thirds of the retina. 

Lens
Biconvex, avascular, transparent structure.
Suspends behind the iris by the zonules which are connected to the ciliary body.
Serves to converge light onto the retina.
Accommodation
Ciliary muscle constrict > zonular tension decreases > lens becomes more spherical > more dioptric power that converge light from a near target onto the retina.
Presbyopia
Cataract
With age, lens fibers are less transparent – cataract
Lens is removed and a plastic lens is inserted instead
(3)Inner layer
The retina
The innermost layer of the eye, comparable to the film inside of a camera. 
It is composed of nerve tissue which senses the light entering the eye. 

Retinal gross anatomy
  • Fovea: area with the highest concentration of photoreceptors.
  • Central retina: A circular field of approximately 6 mm around the fovea.
  • Peripheral retina: stretching to the ora serrata.

  • Rods and cones

Cones
  • Concentrated in the fovea
  • Most active in daylight
  • Central vision
Rods
  • Mostly in the peripheral retina
  • Most active in night vision
  • Peripheral vision
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Optic nerve

Consists of 1 million axons that arise from the retina.
Leaves the eye through the sclera – optic canal – into the cranium.

Oculomotor nerve
Originates from the brainstem – cavernous sinus – divides into superior and inferior divisions.
  1. Superior division: supply the superior rectus and levator palpebrae.
  2. Inferior division supplies the inferior and medial rectus and inferior oblique muscle.
  3. In addition the oculomotor supplies the pupillary muscles. 

Oculomotor palsy
    • Exotropia
    • Hypotropia
    • Pupil dilation
    • Ptosis
Trochlear nerve
Dorsal surface of the brain stem – decussates – cavernous sinus – orbit – supplies the superior oblique.
Trochlear palsy
  • Hypertropia
  • Head tilt
Abducens nerve
Originates between the pons and medulla – cavernous sinus – orbit – lateral rectus
Abducens palsy
  • Esotropia
Nystagmus
Involuntary movement of the eyes
May seriously reduces vision
Etiology:
  • Congenital
  • Vestibular
  • Neurologic
  • Vestibulo-ocular reflex
Primitive reflex which aims to keep the eyes on a target when the head moves
The extra-ocular muscles receive information from the otolit organs and move the eyes in the opposite direction

Optics of the Eye
The eye is like a camera. Light must have a clearly pathway to be clearly focused on the sensory receptors of the retina, i.e., Clear cornea, anterior chamber, lens and vitreous cavity.          
The Refractive power of the eye is about  ± 58 dioptres.

VISION
The retina:
            - The central retina contains yellow pigment, Xanthophyll, the so called macula lutea ( yellow spot).
            - It is divided into retinal pigment epithelium & neurosensory retina.
            - Photoreceptors contains visual pigment which consists of a large protein (opsin) attached to retinal (vitamin A aldehyde).
Light splits the opsin from the retinal with initiation of a graded electrical potential → Transmitted through the visual pathway to be processed in the visual    cortex (occipital lobe) → vision sense.

Visual Pathway: Three neurons
            1. Bipolar cell, lies within the retina.
            2. Ganglion cell, synapse in lateral geniculate body.
            3. Third neuron terminates in visual cortex. 


LACRIMAL SYSTEM

Lacrimal system consists of :
1.Glands—which secrete the tears.
2.Passages—which drain the tears.

 Glands:-
1,Lacrimal glands-located in the upper outer portion of the orbit
2,Accessory lacrimal glands-located in the stroma of the conjunctiva mainly at the fornices.

Passage:-
    1, Punctii
    2,Canaliculi
    3,Lacrimal sac
    4,Nasolacrimal duct

Precorneal tear film

1,Inner mucin layer---produced by goblet cells in the conjunctiva. It decreases the surface tension so helps in spreading of aqueous layer.

2,Middle aqueous layer---produced by main lacrimal gland and the accessory lacrimal glands of conjutiva.It lubricates the cornea and conjunctiva.

3,Superficial oily layer---produced by sebaceous glands present in the tarsal plates of the eye lids. It prevents the evaporation of the aqueous layer.
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Mechanism of tear drainage

  • Lids close in a zippary action.
  • Horner’s muscle expands the lacrimal sac while blinking and creats –ve pressure.
  • Punctii dip in the conjunctival sac thus tears are sucked in.
  • Relaxation of muscle pushes the fluid down in to the inferior meatus of the nose.

Functions of precorneal tear film

   1,Maintenance of transparency of cornea.
   2,Making the anterior corneal surface smooth and regular for refraction.
   3,Antibiotic effect.
   4,Drain the FB.

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