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Eye ball consists
of three layers:
1. External
layer-Sclera ;
2.
Middle layer-Choroid ;
3.
Innermost layer-Retina ;
Sclera:
Consists of
dense fibroelastic tissue and varies in thickness, being thickest
posteriorly and thinnest at the corneal equator of the globe. Sclera
is penetrated by the optic nerve and blood vessels.
Uvea (choroid , ciliary body, iris):
(i) Choroid:
Choroid consists of vascular
connective layer with heavily pigmented melanocytes and lies between
sclera externally and retina internally.
(ii) Ciliary body:
Ciliary body is the forward
continuation of the choroid layer bulging into the eye and is
attached to the cornea by suspensory ligament. Ciliary body
continuously produces aqueous humor, which circulates into the
anterior chamber via pupil to be reabsorbed into the canal of
Schlemm. Balanced rates of secretion and reabsorption of aqueous
humor maintains a constant intra-ocular pressure, which stabilizes
the lens and cornea.
(iii)
Iris:
Iris is the most anterior part of
uvea forming diaphragm in front of the lens. It consists of
myoepithelial cells (dilator pupillae) supplied by sympathetic
nervous system and smooth muscle fibres (constrictor pupillae)
supplied by parasympathetic nervous system. Colour of the iris
depends on the amount of pigment in the connective tissue stroma.
Blue eyes contain little stromal pigment whereas brown eyes have
much stromal pigment.
Image
Retina:
Retina is the photosensitive layer
terminating behind the ciliary body and consisting of neurons
arranged in distinct layers.
The outer most layer of retina is
composed of pigmented epithelial cells.
Photoreceptor cells (rods and cones)
forms the second layer.
The rod cells (outer segment)
contains photoprotein, rhodopsin.
Sharp, clear, ‘straight-ahead’ vision is processed by
the macula, which is the
central part of the retina.
The pit or depression within the macula, called the
fovea, provides the
greatest visual acuity.
The choroid
layer contains blood vessels that nourish the retina.
Bruch’s membrane
provides support to the retina.
The
retinal pigmented epithelium (RPE) protects and nourishes the
retina, removes waste products, and prevents new blood vessels from
growing into the retina.
Afferent nerve fibres from the retina
converge to form the optic nerve.
Cones are similar in basic structure
of rods, contain pigments similar to rhodopsin, receptive to
blue, green and red light, and the mechanism of transduction is also
similar.
Image
 Conjunctiva:
Conjunctiva is the epithelium which
covers the exposed part of the sclera and inner surface of the
eyelids.
It is composed of stratified squamous epthelium with goblet
cells in the surface layer.
The conjunctival mucous secretions act
as the protective layer on the exposed surface of the eye.
 Eyelid:
Each
eyelids consists of fibroelastic plate , the tarsus covered
externally by thin, highly folded skin and internally by smooth
conjunctiva. The skin contains hair follicles, Meibomian gland,
glands of Zeis (modified sebaceous glands), and glands of Moll
(modified apocrine sweat glands). Glands of eyelid produce an oily
layer, which covers the tear layer preventing evaporation of the
tears.
Skeletal muscle, orbicularis oculi
(and levator palpibrae in the upper eyelids) represents a forward
continuation of the sub-aponeurotic layer of the scalp.
Blood or inflammatory exudates
collecting above the scalp aponeurosis may track forward causing
marked swelling of the eyelid.
Image
Cornea:
The cornea is the thick, transparent
portion of the sclero-corneal layer enclosing the anterior one-sixth
of the eye.
It is vascular and consists of five
layers.
From outside inwards:
(i) Stratified squamous epithelium,
(ii) Bowman’s membrane (basement
membrane),
(iii) Substantia propria (collagenous
connective tissue forming thin lamellae),
(iv) Descemet’s membrane (thick
elastic basement membrane), and
(v) Endothelium lining the inner
surface of cornea.
Lacrimal gland :
Image1
;
Image2
Lacrimal
gland secretes tears, a watery fluid containing antibacterial enzyme
lysozyme and electrolytes. Each gland drains into the superior
fornix and tears drain to the inner aspect of the eye and then into
the nasal cavity via the nasolacrimal duct.
Related External Link
:
click1
; click2
.
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