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Term Paper on the Human Eyeball
Term Paper # 1. Meaning and Parts of the Human Eyeball:
It is a highly differentiated end organ, is the organ of sight and one of the five special senses. It is almost spherical in shape and its diameter is about 2.5 cm. There are two eyeballs, situated within the bony orbital cavity.
Eyeball is made up of three concentric coats:
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1. Outer or Fibrous Coat:
Comprises sclera and cornea.
2. Middle or Vascular Coat:
Also called uveal tract, consists of the choroids, ciliary body and iris.
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3. Inner or Nervous Coat:
It is the retina. Eyeball is a cystic structure kept distended by pressure inside it. It has a tough fibrous coat and a fluid filled cavity that maintains the shape and distributes the hydraulic pressure uniformly. It lies embedded in an orbital pad of fat inside the orbital cavity, covered by “Tenon’s capsule”.
It is made up of two segments:
a. Anterior 1/6th is segment of a small sphere, i.e., Cornea.
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b. Posterior 5/6th is segment of a large sphere, i.e., Sclera.
Antero posterior diameter is about 24 mm (vertical).
Transverse diameter is about 23 mm or equal.
Light entering the eyeball passes through several refracting media. From before backwards these are – Cornea, aqueous humour, the lens, the vitreous body and retina.
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1. The Outer Fibrous Coat:
Consisting of a cuplike expansion of dural sheath of optic nerve. It is tough fibrous tunic formed by sclera and cornea. Sclera is opaque and forms posterior 5/6th part.
(A) Cornea is transparent and forms anterior 1/6th part of the eyeball, non-vascular and is nourished by lymph which circulates in the numerous corneal spaces, i.e., aqueous humour. It is supplied by branches of ophthalmic nerve (through ciliary ganglion) and the short ciliary nerves. Pain is the only sensation aroused from the cornea.
The diameter of cornea is about 11 mm.
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It forms refractive media of the eye and made up of five layers:
(i) Corneal epithelium
(ii) Bowman’s membrane
(iii) Substantia propria
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(iv) Descemet’s membrane
(v) Endothelium.
It is lined by stratified squamous non-keratizined epithelium externally continuous with conjunctiva and endothelium lining the anterior chamber internally.
Cornea is thicker peripherally and thinner centrally.
At the periphery, the cornea meets the sclera at sclero-corneal junction called limbus – on the inner aspect lies a circular canal known as sinus venosus sclerae or canal of schlemn, which drains excessive amount of aqueous humour.
Applied Anatomy:
1. Keratitis:
Inflammation of cornea.
2. Leucoma:
A white scar on cornea.
3. Exposure Keratitis:
Epithelium of the cornea becomes dry and hazy due to exposure.
4. Arcus Senilis:
It is a lipoid degeneration of the corneal border in elderly individuals.
5. Corneal Transplantation:
(B) Sclera (Skleros = hard) is opaque and forms posterior 5/6th of the eyeball. It is made up of dense fibrous tissue which is firm white and maintains the shape of the eyeball.
Its average thickness is about 1 mm and covered by membrane called Tenon’s capsule.
The anterior part is covered by conjunctiva and is white.
The sclera is almost avascular. However, the loose connective tissue between conjunctiva and sclera called as episclera is vascular.
The recti and oblique muscles of the eyeball are inserted over the sclera.
Lamina cribrosa is situated on the posterior surface of the sclera and is pierced by optic nerve fibres.
Posteriorly sclera is continuous with the dural sheath of the optic nerve.
Anteriorly it is continuous with the cornea at the sclero corneal junction.
Sclera is pierced by the following vessels and nerves:
1. A pair of long posterior ciliary arteries.
2. Short posterior ciliary arteries about 6 to 9 in number.
3. Long and short ciliary nerves.
4. Long ciliary arteries.
5. Venae verticosae – about 4 in number.
6. Anterior ciliary arteries about 7 in number.
7. Optic nerve.
Applied Anatomy:
a. Staphyloma:
Localised bulging of sclera.
b. Blue Sclera:
Congenital condition – bluish discolouration of the sclera due to thinning of sclera.
c. Cupped Disc:
Due to increased intra cranial tension lamina cribrosa will buldge outwards – Papilledema.
d. Scleritis:
Inflammation of sclera.
2. Vascular Pigmented Coat (Uveal Tract):
It has three parts – choroid, ciliary body and iris diaphragm. Iris has an aperture in its centre called pupil. It is formed by expansion of archnoid mater and piamater. It is vascular and pigmented layer.
(a) Choroid:
It is a soft thin pigmented membrane. Anteriorly it ends at the ora serrata by merging with the ciliary body; posteriorly it is perforated by optic nerve.
It is 0.2 mm thick.
The choroid supplies nutrition to the retina.
It shows blood vessels arranged in three layers. All layers are held together by connective tissue stroma which contains pigmented chromatophores.
It has rich sensory nerve supply from naso ciliary nerve.
(b) Ciliary Body:
It is the anterior continuation of the choroid upto limbus which lies between choroid and iris. It is triangular shaped in cross-section. It has a base and an apex. To the middle of the base iris is attached. The apex is continuous with the choroid.
It consists of three parts:
(i) Ciliary Ring:
Flattened circular band.
(ii) Ciliary Processes:
60 to 80 in number, formed by inward folding of layer of choroids – connected with suspensory ligament of lens.
It secretes aqueous humour.
(iii) Ciliary Muscles:
Ciliary muscles are plain muscles on outer side of choroid run from base to limbus.
Outer fibres called Brucke’s muscle, run antero posterior.
Inner fibres are called Muller’s muscle run, circularly.
Action:
The contraction of the muscle relaxes the suspensory ligament so that lens becomes more convex and helps in accommodation and are also responsible for opening the canal of schlemn and helps in the drainage of the aqueous humour.
Nerve Supply of Muscle:
Parasympathetic fibres coming from Edinger-Westphal nucleus, pass through oculomotor nerve are relayed in the ciliary ganglion to supply ciliary muscles.
C. Iris:
It is pigmented diaphragm. Iris is the anterior part of the uveal tract. It forms a circular curtain with an opening in the centre called pupil. It is hanging down from the ciliary body. It lies between the cornea and lens, i.e., with in the anterior compartment and divides it into anterior and posterior chambers.
The space between lens and iris is called posterior chamber. The space anterior to the iris is called anterior chamber. The two chambers communicate through the pupil. Both chambers containing aqueous humour secreted by ciliary processes. Pupil controls the amount of light entering the eye.
It is made up of four layers:
1. Endothelial layer.
2. Stroma is formed by connective tissue, containing pigment loaded chromatophores, vessels and nerves. Vessels are arranged in a radiating fashion, near the pupil they anastomose.
3. Muscles of the iris- Circular muscles forming sphincter pupillae and radial muscles forming dilator pupillae.
4. Epithelial layer- Covers the back of the iris are pigmented epithelial cells.
Blood Supply to Iris:
Greater arterial circle of iris is situated in the ciliary body, formed by pair of long posterior ciliary arteries.
Nerve Supply:
Sphincter pupillae is supplied by parasymphathetic fibres from oculomotor nerve (Edinger-Westphal nucleus).
Dilator Pupillae:
Supplied by sympathetic fibres from T1 ganglion.
Action:
Sphincter pupillae – constricts the pupil during bright light.
Dilator pupillae dilates the pupil during dim light.
Applied Anatomy:
a. Iritis:
Inflammation of the iris.
b. Cyclitis:
Inflammation of ciliary body.
c. Iridodialysis:
Tear of iris at its ciliary attachment.
d. Synechiae:
Adherence of the iris to the cornea or lens.
3. The Retina:
It is also called as nervous coat, and is photosensitive layer of eye. It has an outer pigmented layer and an inner layer of nervous tissue.
Anteriorly retina ends at the ora serrata.
Retina is situated between the hyaloid membrane and vitreous body internally and choroid externally.
Macula lutea is a yellow spot situated at the posterior pole of the eye. It is about 1 to 2 mm in diameter. It is the site of maximum acuity due to collection of cones.
The optic disc is situated 3 mm medial to the maculalutea. Disc is slightly depressed and is pierced by the central artery of the retina. Rods and cones are absent in the optic disc. It is insensitive to light and is known as blind spot.
Microscopically retina has ten layers from without inwards:
(i) Layer of pigmented epithelim
(ii) Layer of rods and cones
(iii) External limiting membrane
(iv) Outer nuclear layer
(v) Outer plexiform layer
(vi) Inner nuclear layer
(vii) Inner plexiform layer
(viii) Ganglion cell layer
(ix) Layer of nerve fibres
(x) Internal limiting membrane.
Colour Vision:
Cones of the retina are responsible for colour vision. Rods may perceive blue colour.
Blood Supply:
i. Chorio capillaries supply outer layers of retina.
ii. Central artery of retina supplies inner layers.
Venous Drainage:
Blood from retina is drained into choroidal veins and central vein of retina. Which drains into Caverneous Sinus.
Term Paper # 2.
Segments and Chambers of the Human Eyeball:
Eyeball can be divided into two segments anterior and posterior.
1. Anterior Segment:
It lies anterior to lens which is suspended from the ciliary body by zonules, i.e., suspensory ligament. Structures anterior to lens – are iris cornea and two aqueous filled spaces, i.e., anterior and posterior chambers.
(a) Anterior Chamber:
It is bounded anteriorly by cornea and posteriorly by the iris and part of ciliary body. It communicates with posterior chamber through pupil its peripheral recess is called the angle of the anterior chamber irido – corneal angle formed by trabecular meshwork. Next to it canal of schlemn is present in the substance of sclera. Aqueous humour produced by ciliary processes is drained from the anterior chamber through this meshwork and canal.
(b) Posterior Chamber:
It is present behind the iris and in front of lens. It is a triangular space containing aqueous humour.
2. Posterior Segment:
It is present behind the lens and infront of retina and optic disc, filled with vitreous humour. It is transparent, colourless and jelly like, consisting of 99% of water with small amount of mucoprotein.
Blood Supply:
The short posterior ciliary arteries divide into 10 to 20 branches, pierce the sclera and supply the choroid and sclera. Two long posterior ciliary arteries supply the ciliary body and iris. They reach the ciliary muscle and divide into two branches which enter the substance of muscle at its anterior end and anastomose with anterior ciliary arteries – form circulus iridis major and supply ciliary body and iris.
Applied Anatomy:
a. Retinitis – inflammation of retina.
b. Sudden occlusion of the central artery of retina causes blindness.
c. Thrombosis of retinal vein – occur in elderly.
d. Pigmentosa – Degenerative disease. Night blindness and deposition of melanin in the retina.
e. Retinal detachment – separation of retina from choroid.
Term Paper # 3.
Movements of the Human Eyeball:
Occurs on three axis:
I. Vertical Axis of Movement:
(a) Adduction:
Main adductor is medial rectus.
Accessory adductors are superior rectus and inferior rectus.
(b) Abduction:
Main abductor is lateral rectus.
Accessory abductors are superior oblique and inferior oblique.
II. Transverse Axis Movements:
(a) Elevation:
Superior rectus and inferior oblique.
(b) Depression:
Inferior rectus and superior oblique.
III. Antero Posterior Axis Movements:
(a) Intorsion:
Superior rectus and superior oblique muscle.
(b) Extorsion:
Inferior rectus and inferior oblique muscle.
Term Paper # 4.
Contents of the Human Eyeball:
1. Aqueous Humour:
It is a clear fluid secreted by ciliary process of the ciliary body in the posterior chamber, passes through pupil and enters the anterior chamber. From here it is drained away through the spaces at the irodo corneal angle into the canal of schlemn, passes away through venae verticosae.
When there is obstruction in the circulation – intra ocular pressure is raised – causes glaucoma – it is a severely painful condition.
2. The Lens:
It is a transparent biconvex structure, forms one of the constituents of the refractive media of the eye. Its main function is to converge light rays and form images on the retina.
Diameter is about 9 mm.
Thickness is about 4 mm.
It is suspended by the suspensory ligament.
The posterior surface is more convex than anterior surface.
It is covered by an elastic capsule.
Nutrition of the lens is provided by:
a. Aqueous humour present in the anterior chamber.
b. Auto oxidation system within the lens.
3. Accommodation:
Ability of the eye to adjust for both distant and near vision.
It is done by the following mechanisms:
a. Contraction of ciliary muscle.
b. Choroid is pulled forwards and inwards.
c. Relaxation of the suspensory ligament caused by ciliary process.
Accommodation Reflex:
It is constriction of pupil while looking at a near object.
Pathway for Accommodation Reflex:
Visual receptors (rods and cones) → bipolar cells of retina → ganglion cells → optic nerve → optic chiasma → optic tract → lateral geniculate body → optic radiation → visual area of occipital lobe (area 17) → frontal lobe through the association fibres → third nerve nucleus → oculomotor nerve → ciliary ganglion → short ciliary nerves → ciliary muscles.
Applied Anatomy:
(i) Cataract – lens become opaque.
(ii) Congenital cataract – since after birth.
(iii) Senile cataract – due to old age.
(iv) Extraction of lens – may be extra ocular or intra ocular.
4. The Vitreous Body:
This is a jelly-like body with in the posterior compartment of the eye – lies behind the lens. It is transparent and enclosed by the hyaloid membrane. Hyaloid canal is a passage in the central part of the vitreous, extends from the posterior surface of the lens to the optic disc. In the fetus hyaloid artery passes through it, later on it oblitrates.
It forms one of the refractive media of the eye.
Development:
Optic vesicle forms optic cup. It is an out-growth from the forebrain vesicle.
Lens:
Develops from lens placode (Ectodermal in origin).
Retina:
Pigment layer from the outer layer of optic cup and nervous layers from the inner layer of optic cup.
Choroid and Sclera:
Develops from mesoderm.
Cornea:
This develops from surface ectoderm.