09 Control and Coordination - part 12 - Eye

 

09 Control and Coordination - part 12 - Eye

Eye :

• Are a pair of sensory organs of vision. 
• Located in the orbit of skull with a cushion of fat around them. 
• Each eye is spherical/rounded and called eyeball. 
• Protected bones, eyebrows, upper and lower eyelids with eyelashes and the lacrimal/ tear glands. 
• Movement of the eyeball within the orbit is controlled by 6 sets of muscles.
• Wall of the eyeball is made up of 3 layers-
  

1. Sclera
2. Choroid and
3. Retina

1. Sclera/scelrotic :  

• Outermost layer made of dense fibroelastic connetive tissue with collagen fibres. 
• Provides attachment to the eyeball muscles. 
• The anterior thick, transparent part of sclera is cornea. 
• It is slightly bulged out for focussing light on the retina. 
• Provided with blood vessels, however the cornea is devoid of them. 
• Cornea is nourished by aqueous humour and also by lacrimal secretion. 
• The exposed part of sclera and the entire Cornea are covered by a transparent membranous covering called conjuctiva. 
• It provides protection and lubrication to the cornea.

2. Choroid /Uvea : 

• Middle, vascular and pigmented layer. 
• It is not a complete layer and can be divided into 3 regions - 

1. Choroid proper 
2. Ciliary body 
3. Iris

a. The choroid proper : 

• Lines the sclera.
• Due to its pigmented nature it prevents internal reflection. 
• The blood vessels of choroid provide nutrition and oxygen to the retina.

b. Ciliary body : 

• Thick, muscular, ring like structure at the junction of choroid and iris. 
• Its epithelium secretes aqueous humor.
• Attached to the ciliary body are suspensory ligaments which hold the lens. 
• Ligaments and muscles of the ciliary body help in the adjustment of the size of lens.

c. Iris : 

• At the junction of the sclera and cornea, the vascular part of choroid sharply bends into the cavity of eyeball, forming a thin and coloured partition called iris. 
• Perforated in the middle by an aperture called pupil. 
• Smooth muscles of the iris help in regulating the size of pupil depending on the intensity of light entering the eyeball. 
• The pigment in the iris determines the colour of the eye.

Lens : 

• Transparent, elastic, biconvex structure. 
• Suspended in the eyeball by the suspensory ligaments. 
• The lens and suspensory ligaments divide the cavity of the eyeball into a small anterior aqueous chamber, filled with a clear watery fluid aqueous humor and a posterior large vitreous chamber, filled with a jelly like vitreous humor. 
• Maintains shape of the eyeball.
• Maintain pressure for keeping the lens in position.

3. Retina : 

• Innermost, delicate, non vascular light sensitive layer.
• Has 2 regions - 

1. Single layer of pigmented non sensory part lining the iris and ciliary body 
2. Sensory part lining the choroid. 

• Has an outer pigmented part and an inner nervous part. 
• The inner nervous part is transparent and made of 3 layers -

1. Outer photosensitive layer made of rod and cone cells. 
2. Middle layer of bipolar nerve cells 
3. Inner layer of ganglion cells. 

• The nerve fibres from the basal end of the ganglioncells collectively form the optic nerve.

Do you know ?

• The glowing of eyes in some animals is due to presence of a reflecting layer behind the retina, called tapetum lucidum.

Blind spot :

• An area diagonally opposite to the lens. 
• Area of retina from where the optic nerve and blood vessels leave the eyeball. 
• No rod and cone cells in this region. 

Macula lutea

• An area, lateral to the and above the blind spot is called yellow area or macula lutea. 
• At its centre is a depression called lovea centralis. 
• It has maximum density of cone cells and is the place of formation of sharpest vision.
• Rod and cone cells lie deep in the retina, so that light has to pass through the ganglion and bipolar cells before reaching them.

Photo receptor cells : 

• These are of two types-

1. Rod cells 
2. Cone cells.

• They contain light sensitive proteins termed as photopigments. 
• Cones are responsible for daylight (photopic) vision and colour vision. 
• Rods function in dim light (Scotopic) vision. 
• The purple red protein called rhodopsin is present in the rods which is vitamin A derivative. 
• Cones are of three types, which contain their own characteristic photo-pigments that respond to red, green and blue lights. 
• Various combinations of these cones and their photopigments produce sensation of different colours. 
• Sensation of white light is produced due to the simultaneous equal stimulation of these three types of cones.
• The Optic nerve consists of the fibres arising from the base of ganglion cells. 
• It leaves the eye ball from the posterior side and carries visual impulses from the retina to the brain.

Generation of image :

• Light rays from the object pass through the conjunctiva, cornea through the pupil upon the lens and is focused on the retina to form an image. 
• In the visual area of cerebrum, the nerve impulses are analysed and the image formed is recognized.
  

Always Remember
Accomodation : 

• Lens makes fine adjustments to bring a sharp focus on retina.
• Ability of the lens by which the light ray from far and near objects is focused on the retina is called accomodation power of the lens.