The eye is the visual organ. It resides in the orbital cavity and is supplied by the optic nerve (the second cranial nerve). It is a sensory unit composed of three parts: receptor, sensory pathway, and a brain center.
· It is spherical structure situated in the orbital cavity
· It is about 2.5 cm in diameter in shaped
· Adipose tissue occupies the space between the eye and orbital cavity.
· The skeletal walls of the orbit and the fat protect the eye from harm.
Accessory organs & Eye Protection
· Orbital cavities (bony sockets) – house & protection of eye
· Adipose tissue – cushions the eye
ü The lacrimal glands, each about the size and shape of an almond.
ü Secrete tears that lubricate & have a germicidal effect.
ü The lacrimal fluid drains into 6–12 lacrimal ducts that empty tears onto the surface of the conjunctiva of the upper lid.
ü The tears pass medially over the anterior surface of the eyeball to enter two small openings called lacrimal puncta.
FLOW OF TEARS
LACRIMAL GLAND secretes tears into
LACRIMAL DUCTS, which distribute tears over surface of eyeball
SUPERIOR OR INFERIOR LACRIMAL CANALICULI drain tears into
LACRIMAL SAC, which drains tears into
NASOLACRIMAL DUCT, which drains tears into
· Protect against foreign articles, perspiration, & direct rays of light
· Folds of skin that cover the surface of the eye; close by reflex action when an object approach.
· Secrete oils that prevent lids from sticking together.
05│Muscle of eye
· Extrinsic muscles – muscles located outside of the eye that control certain eye movements such as moving the eyeball from side to side or rolling the eyes.
· Muscles located inside the eye that help hold the lens in place & modify its shape.
Anatomy of the eyeball
Anatomically, the wall of the eyeball consists of three layers:
(1) fibrous tunic, (2) vascular tunic, and (3) retina (inner tunic).
01│ Fibrous tunic:
It is the superficial layer of the eyeball and consists of the anterior cornea and posterior sclera.
ü It is a curved and transparent coat that covers the colored iris.
ü The cornea helps focus light onto the retina.
ü The outer surface of the cornea consists of stratified squamous epithelium (nonkeratinized).
ü Middle surface consists of collagen fibers and fibroblasts, and
ü Inner surface is simple squamous epithelium.
ü The white part of the eye and consist of dense connective tissue made up mostly of collagen fibers and fibroblasts.
ü It coves entire eyeball except cornea.
ü It gives shape to the eyeball.
ü At the junction of the sclera and cornea is an opening known as the scleral venous sinus or (canal of Schlemm).
Canal of Schlemm
It is a unique vascular structure that functions to maintain fluid homeostasis by draining aqueous humor from the eye into the systemic circulation.
02│ Vascular Tunic:
It is the middle layer of the eyeball. It is composed of three parts: choroid, ciliary body, and Iris.
Choroid: posterior portion of the vascular tunic
ü Middle layer of the eye; it supplies blood vessels to the eye.
ü The choroid contains melanocytes that produce the pigment melanin, which causes this layer to appear dark brown in color.
ü The dark pigment granules that prevent the reflection of light in the eye.
Ciliary body: anterior portion of the vascular tunic
ü Ciliary body consists of ciliary processes and ciliary muscle.
ü Ciliary processes: contain blood capillaries that secrete aqueous humor. Extending from the ciliary process are zonular fibers or suspensory ligaments that attach to the lens.
ü Ciliary muscle: it is a circular band of smooth muscle. Contraction or relaxation of the ciliary muscle changes the tightness of the zonular fibers. which alters the shape of the lens, adapting it for near or far vision.
ü The colored portion of the eyeball.
ü It consists of melanocytes and circular and radial smooth muscle fibers.
ü The amount of melanin present in the iris determines the eye color.
ü A principal function of the iris is to regulate the amount of light entering the eyeball through the pupil.
ü Rounded opening of the iris through which light passes.
· High: brown to black
· Moderate: green
· Low: blue
Responses of the pupil to light
ü Contraction of the circular muscles causes constriction of the pupil.
ü Contraction of the radial muscles causes dilation of the pupil.
ü Parasympathetic stimulation constricts the pupil and sympathetic stimulation dilates.
The third and inner layer of the eyeball. It is an extremely delicate structure and is well adapted for stimulation by light rays. The light sensitive layer consists of sensory receptor cells, rods and cones, which contain photosensitive pigments that convert light rays into nerve impulses.
ü Three distinct layers of retinal neurons
1. The photoreceptor layer
2. The bipolar cell layers
3. The ganglion cell layer
The photoreceptor layer: the primary light sensing cells in the retina
There are two types;
1. Rods (6 million)
ü Rods do not provide color vision.
ü It allows us to see in dim light (we can see only black, white and all shades of grey color).
2. Cons (120 million)
ü Cons produce color vision (stimulate brighter light)
ü Three types of cones are present in the retina.
o Blue cones – sensitive to blue light
o Green cones – sensitive to green light
o Red cones – sensitive to red light
The information flows through
ü The axons of ganglion cells extend posteriorly to the optic disc and exit the eyeball as the optic (II cranial nerve) nerve.
ü The optic disc is also called the blind spot.
ü It contains no rods or cones, we cannot see images that strike the blind spot.
ü The lens is present behind the pupil and iris, within the cavity of the eyeball.
ü Within the lens cells contain proteins called crystallins.
ü It arranged like the layers of an onion.
ü It normally transparent and having lack of blood vessels also enclosed by clear connective tissue.
ü Contains aqueous humor that helps maintain shape of eyeball and supplies oxygen and nutrients to lens and cornea.
ü Contains vitreous body that helps maintain shape of eyeball and keeps retina attached to choroid.
- Refraction of Light Rays
- Accommodation and the Near Point of Vision
- Constriction of the Pupil
01│Refraction of Light Rays
As light rays enter the eye, they are refracted at the anterior and posterior surfaces of the cornea.
Both surfaces of the lens of the eye further refract the light rays so they come into exact focus on the retina.
Images focused on the retina are inverted.
About 75% of the total refraction of light occurs at the cornea.
The lens provides the remaining 25% of focusing power and also changes the focus to view near or distant objects.
(object is away from the viewer – nearly parallel rays; object is closer – divergent rays)
This process of reflection is called accommodation.
02│Accommodation and the Near Point of Vision
ü The surface of a lens is convex, that lens will refract incoming light rays toward each other, so that they eventually intersect.
ü When the eye is focusing on a close object, the lens becomes more curved, causing greater refraction of the light rays.
ü This increase in the curvature of the lens for near vision is called accommodation.
ü When the eye is focusing on a distance object, the ciliary muscle of the ciliary body is relaxed and the lens is flatter.
03│Constriction of the Pupil
ü Constriction of the pupil is a narrowing of the diameter of the hole through which light enters the eye due to the contraction of the circular muscles of the iris.
ü This autonomic reflex occurs simultaneously with accommodation and prevents light rays from entering the eye through the periphery of the lens.
Hi….!! My name is Smrutiranjan Dash, From Odisha, India. Professionally I am Assistant Professor at The Pharmaceutical College, Barpali, Odisha, department of Pharmacology.