Eye : Human have binocular vision. The eye can discriminate colour, appraise length, width and depth visually and form true inverted image.
# (i) Structure of eye : The eyes are two in number and lodged in orbits (bony socket) of skull. A very small part (front) is exposed while the rest lies in the orbit. The eye is a hollow, spherical organ, about 2.5 cm in diameter and about 6 to 8 gram in weight. It has two parts –
(a) Protective devices
(b) Eye ball
# (a) Protective devices : Eye has four protective devices eye brow, eye lids, eye lashes, eye glands and adipose tissue.
- (1) Eye brows : The outwardly directed hair of the eyebrows carry the sweat and rain drops trickling down the forehead to the sides to prevent their falling into the eyes.
- (2) Eye lids (Palpebrae) : In man two eyelids are present, upper is movable. They are regularly closed at short intervals to clean the cornea. This is called blinking. In frog out of two upper eyelid is immovable and lower eyelid is movable. Nictitating membrane is present in frog which protect eye in water. Movement of nictitating membrane takes place by retractor bulbi. It becomes folded by levator bulbi.
In rabbit upper eye lid is movable. Hence nictitating membrane is reduced, as in most other mammals, simply to a small fold at the inner angle of an eye. Normally it remains retracted, but can be spread over the conjunctiva when required. A nonfunctional vestigeal nictitating membrane, called plica semilunaris, occurs in human eyes. It remains permanently retracted at the inner angle of each eye.
- (3) Eye lashes : The eyelids bear at the free edge a row of stiff hair, the eye lashes. These check the entry of dust particles, tiny insects and rain drops into the eyes.
- (4) Eye glands
Meibomian gland : The eye-lids bear at the free edge a row of meibomian gland that is modified sebaceous gland. (Act as a lumbricant).
Lachrymal gland or Tear gland : It lies in the upper outer part of the orbit and secretes a slightly saline, watery fluid that contains a bacteriolytic enzyme named lysozyme. This secretion moistens the surface of the eyeball. The excess of this secretion passes through nasolachrymal duct.
Harderian gland : Some aquatic mammals (whale) possess harderian gland which lumbricate nictitating membrane. It is also found in frog and birds.
Glands of Moll : It is modified sweat gland and open into the follicles of eyelashes.
In human meibomian lachrymal and Moll's glands are present.
- (5) Connective tissue : A layer of fatty connective tissue surrounds the eyeball. It serves as a soft shockproof pad.
(b) Eye ball : Eye ball is made up of 3 coats
Outer fibrous coat (Sclerotic layer), middle vascular coat (choroid layer) and inner nervous coat (retina). Of these, the fibrous coat alone is complete. Others are incomplete on the front side.
# (1) Sclerotic layer (Fibrous tunica) : Outer most and opaque, fibrous and non-vascular layer easily seen as white of the eye.
Cornea : In the centre, scleroticlayer it merges with the transparent round window called cornea (or in other words – In exposed central part, sclerotic layer form transparent cornea). It is separated from sclera by schlemm's canal. The front part of the cornea is highly curved and convex acting as a lens and forms the first of the refracting surface of the eye.
Conjunctiva : The cornea and exposed part of sclera are covered externally by a thin, transparent membrane the conjunctiva.
The latter is composed of a stratified epithelium and is continuos with the epidermis that lines the eyelids. The conjunctiva, thus has 2 parts : Occular conjunctiva that covers the front of the eye and palpebral conjunctiva which lines the eyelids. (In man conjunctiva is fused with corneal layer). In sore or "pink" eyes, the conjunctiva gets inflammed, causing conjunctivitis.
# (2) Choroid layer (Vascular tunica) : Also known as uvea middle. it is vascular layer which supplies nutrients to the eye. It is distinguished into three parts choroid, ciliary body and iris.
Choroid : The choroid occurs in the main part of eye ball adhered to the sclerotic. Its inner surface (close to the retina) contains pigment cells that absorb scattered light and so increase the sharpness of focussed image. (The pigment is reddish in rabbit and black, brown or bluish in man). In front it form ciliary body.
Ciliary body : Ciliary body is vascular and pigmented like choroid, but it also contains a number of circular, radial and oblique smooth ciliary muscles. The ciliary body is hidden by iris. The ciliary body helps in accommodation by altering the focus of eye from object. (Ciliary body produce aqueous fluid)
Iris : Beyond the ciliary body, the vascular tunic sharply turns inwards, forming a circular, shelf-like diaphragm called iris. The large central aperture of iris is called pupil. In pupil circular and radial muscles are present. Pupil dilate by the contraction of radial muscle and constrict by the contraction of circular muscle. The colour of the iris is responsible for colour of eye e.g. brown, black, blue or green. In albinos, iris is deficient of pigments.
Eye : Human have binocular vision. The eye can discriminate colour, appraise length, width and depth visually and form true inverted image.
# (i) Structure of eye : The eyes are two in number and lodged in orbits (bony socket) of skull. A very small part (front) is exposed while the rest lies in the orbit. The eye is a hollow, spherical organ, about 2.5 cm in diameter and about 6 to 8 gram in weight. It has two parts –
(a) Protective devices
(b) Eye ball
# (a) Protective devices : Eye has four protective devices eye brow, eye lids, eye lashes, eye glands and adipose tissue.
- (1) Eye brows : The outwardly directed hair of the eyebrows carry the sweat and rain drops trickling down the forehead to the sides to prevent their falling into the eyes.
- (2) Eye lids (Palpebrae) : In man two eyelids are present, upper is movable. They are regularly closed at short intervals to clean the cornea. This is called blinking. In frog out of two upper eyelid is immovable and lower eyelid is movable. Nictitating membrane is present in frog which protect eye in water. Movement of nictitating membrane takes place by retractor bulbi. It becomes folded by levator bulbi.
In rabbit upper eye lid is movable. Hence nictitating membrane is reduced, as in most other mammals, simply to a small fold at the inner angle of an eye. Normally it remains retracted, but can be spread over the conjunctiva when required. A nonfunctional vestigeal nictitating membrane, called plica semilunaris, occurs in human eyes. It remains permanently retracted at the inner angle of each eye.
- (3) Eye lashes : The eyelids bear at the free edge a row of stiff hair, the eye lashes. These check the entry of dust particles, tiny insects and rain drops into the eyes.
- (4) Eye glands
Meibomian gland : The eye-lids bear at the free edge a row of meibomian gland that is modified sebaceous gland. (Act as a lumbricant).
Lachrymal gland or Tear gland : It lies in the upper outer part of the orbit and secretes a slightly saline, watery fluid that contains a bacteriolytic enzyme named lysozyme. This secretion moistens the surface of the eyeball. The excess of this secretion passes through nasolachrymal duct.
Harderian gland : Some aquatic mammals (whale) possess harderian gland which lumbricate nictitating membrane. It is also found in frog and birds.
Glands of Moll : It is modified sweat gland and open into the follicles of eyelashes.
In human meibomian lachrymal and Moll's glands are present.
- (5) Connective tissue : A layer of fatty connective tissue surrounds the eyeball. It serves as a soft shockproof pad.
(b) Eye ball : Eye ball is made up of 3 coats
Outer fibrous coat (Sclerotic layer), middle vascular coat (choroid layer) and inner nervous coat (retina). Of these, the fibrous coat alone is complete. Others are incomplete on the front side.
# (1) Sclerotic layer (Fibrous tunica) : Outer most and opaque, fibrous and non-vascular layer easily seen as white of the eye.
Cornea : In the centre, scleroticlayer it merges with the transparent round window called cornea (or in other words – In exposed central part, sclerotic layer form transparent cornea). It is separated from sclera by schlemm's canal. The front part of the cornea is highly curved and convex acting as a lens and forms the first of the refracting surface of the eye.
Conjunctiva : The cornea and exposed part of sclera are covered externally by a thin, transparent membrane the conjunctiva.
The latter is composed of a stratified epithelium and is continuos with the epidermis that lines the eyelids. The conjunctiva, thus has 2 parts : Occular conjunctiva that covers the front of the eye and palpebral conjunctiva which lines the eyelids. (In man conjunctiva is fused with corneal layer). In sore or "pink" eyes, the conjunctiva gets inflammed, causing conjunctivitis.
# (2) Choroid layer (Vascular tunica) : Also known as uvea middle. it is vascular layer which supplies nutrients to the eye. It is distinguished into three parts choroid, ciliary body and iris.
Choroid : The choroid occurs in the main part of eye ball adhered to the sclerotic. Its inner surface (close to the retina) contains pigment cells that absorb scattered light and so increase the sharpness of focussed image. (The pigment is reddish in rabbit and black, brown or bluish in man). In front it form ciliary body.
Ciliary body : Ciliary body is vascular and pigmented like choroid, but it also contains a number of circular, radial and oblique smooth ciliary muscles. The ciliary body is hidden by iris. The ciliary body helps in accommodation by altering the focus of eye from object. (Ciliary body produce aqueous fluid)
Iris : Beyond the ciliary body, the vascular tunic sharply turns inwards, forming a circular, shelf-like diaphragm called iris. The large central aperture of iris is called pupil. In pupil circular and radial muscles are present. Pupil dilate by the contraction of radial muscle and constrict by the contraction of circular muscle. The colour of the iris is responsible for colour of eye e.g. brown, black, blue or green. In albinos, iris is deficient of pigments.