The Sense Organs

The sense organs enable us to be aware of the condition of the environment. A receptor is any specialized tissue or cell sensitive to a specific stimulus. We have five sense organs ~ Eyes, Ears, Nose, Tongue, Skin.

MechanoreceptorsReceptors of touch, i.e. pressure on the skin due to mechanical
ChemoreceptorsReceptors of taste of the tongue and smell of the nose due to chemical influence.
PhotoreceptorsReceptors of light present in rods and cones of the retina of eyes.
ThermoreceptorsHeat and cold receptors in the skin due to change in temperature.

The Eyes ~ sense organs

  • The two eyes are located in deep sockets called orbits.
  • The upper and lower moveable eyelids protect the front surface of the eyes.
  • There are 6–12 tear glands.
  • Functions of the tear glands are
    • Lubricate the surface of the eye
    • Wash away the dust particles
  • A thin membrane which covers the entire front part of the eyes is called conjunctiva.
  • Due to viral infection of the conjunctiva, we suffer from eye disease called conjunctivitis.
  • Also Read The Nervous System

Structure of the Eyeball

The wall of the eyeball is composed of the folowing three concentric layers:

Sclerotic Layer (Outer Layer):

  • The white visible portion of the eyeball is nothing but the sclera.
  • The sclera covers the coloured part of the eye, i.e. the cornea.

Choroid Layer (Middle Layer):

  • Richly supplied with blood vessels to provide proper nourishment.
  • Choroid expands in the front to form a ciliary body.
  • Iris is also a part of the choroid.
  • The iris partially covers the lens. It leaves a circular opening in the centre called a pupil.
  • The muscles of the iris regulate the size of the pupil. Thus, the pupil regulates the amount of light entering the eye.

Retina (Inner Layer):

  • It has two types of sense cells—rods and cones.
  • The rod cells are sensitive to dim light and do not respond to colour.
  • The cone cells are sensitive to bright light and are responsible for colour vision.
  • Also Read Endocrine System

Comparison between Rods and Cones

More in number.Less in number.
Located at the periphery of the retina.Located in the centre of the retina.
Rapid generation of light-sensitive
pigment rhodopsin.
Slow generation of light-sensitive
pigment iodopsin.

Yellow Spot and Blind Spot

Aqueous ChamberVitreous Chamber
Front chamber between the lens and the cornea.Larger chamber behind the lens.
Filled with clear, watery liquid called aqueous humour.Filled with transparent, jelly-like fluid called
vitreous humour.
It refracts light.It protects the retina and its nerve endings.


  • It is transparent, biconvex and crystalline.
  • It is held by a suspensory ligament which attaches the lens to the ciliary body.

Aqueous and Vitreous Chambers

The lens divides the inner cavity of the eye ball into two chambers:

Aqueous ChamberVitreous Chamber
Front chamber between the lens and the cornea.Larger chamber behind the lens.
Filled with clear, watery liquid called aqueous humour.Filled with transparent, jelly-like fluid called
vitreous humour.
It refracts light.It protects the retina and its nerve endings.

Four Major Steps in Seeing an Object:

  1. Light rays reflected from the object enter the eyes through transparent structures.
  2. First, the curvature of the cornea converges the rays to some extent, and then the lens converges them further.
    • The image on the retina is inverted and real.
  3. The light energy produces chemical changes in rods and cones which send the nerve impulse. This nerve impulse is sent to the cerebrum through the optic nerve.
    • The cerebrum gives the sensation of sight.
  4. The brain interprets the inverted image on the retina, and the object is seen upright.

Accommodation Vision

The process of focusing the eyes at different distances is called accommodation. This is brought about by change in the curvature of the lens.

  • For distant vision, the lens is more flattened.
  • The lens remains stretched by the suspensory ligaments.
  • For near vision, the lens becomes convex and rounded.
  • The ciliary muscles contract and pull the ciliary body forward. This releases the tension of suspensory ligaments, making the lens convex and rounded.

Light and Dark Adaptation

Light Adaptation:

When we pass from a dark area to a brightly lit area, we experience a dazzling effect for a short period. This is called light adaptation.

Dark Adaptation:

When we pass from a brightly lit area to a dark area, we experience difficulty in seeing the objects for a short while. This is called dark adaptation.

Common Defects of the Eyes

Myopia (Short-sightedness):

  • Near objects are seen clearly, but distant objects appear blurred.
  • The lens is too curved.
  • Myopia is corrected by suitable concave lens

Hyperopia (Hypermetropia/long-sightedness):

  • Difficulty in seeing nearer objects.
  • The lens is too flat.


  • Some parts of the object are seen in focus, while others appear blurred.


  • Observed in older people. Near objects cannot be seen clearly.


  • The lens turns opaque and the vision is reduced.

Colour blindness:

Colour blind people cannot distinguish between certain colours such as red and green.

Night blindness:

  • Difficulty in seeing in dim light.
  • Due to non-formation of rhodopsin in rod cells.


  • The eyes converge leading to cross eyes.

Corneal opacities:

  • The cornea of patients gets scarred and turns opaque and non-functional
  • Can cause minor irritation, vision problems and even blindness.

Stereoscopic Vision

Humans, monkeys and apes can perceive depth or the relative distance of objects. This is due to simultaneous focusing of an object in both eyes. The images of both eyes are overlapping and give a 3-dimensional effect.


When one looks at a brightly coloured object and then looks at a dark surface, an image of the object in the same colour will persist. This is known as persistence image or after-image.

The Ear ~ sense organs

The human ear has the three following main divisions:

  1. Outer Ear
    • Consists of pinna/auricle and auditory canal.
  2. Middle Ear
    • Contains three ear ossicles—malleus (hammer), incus (anvil) and stapes (stirrup)—and the eustachian tube.
    • The eustachian tube connects the cavity of the middle ear with the throat.
  3. Inner Ear
    • Also known as membranous labyrinth.
    • Contains cochlea and the semicircular canals.
    • The cavity of cochlea is divided into three parallel canals. The middle canal consists of the organ of corti which is responsible for hearing.
    • Ends of the semicircular canals widen to form an ampulla.
    • The ampulla contains sensory cells.
    • The short stem joining the bases of semicircular canals to the cochlea is called the vestibule.
    • The vestibule contains two sacs—utriculus and sacculus.

Functions of the Ear

  1. Hearing
    The pinna collects sound waves and conducts them through the external auditory canal. They finally strike
    on the ear drum and the vibration is set.
  2. Body Balance
    • The sensory cells in the semicircular canals are concerned with dynamic equilibrium, i.e. when the body is in motion.
    • The sensory cells in utriculus and sacculus are concerned with static equilibrium, i.e. when the body is stationary.

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