Sensory Organs in Humans

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Sensory Organs in Humans

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Eye is made up of 3 Layers :

  • Upper Sclerotic
  • Middle Choroids
  • Inner Retina

1. Sclerotic Layer

The Transparent bulging, circular part of sclerotic layer which lies in the front is called cornea.

Cornea is covered by the thin Conjunctiva.

2. Choroid Layer

Choroid layer starts from iris which is next to cornea.

It controls the amount of light entering into the eye (in brighter light, the iris automatically shuts tighter, reducing the amount of light entering the pupil).

The circular aperture of iris is called pupil.

Next is lens (proteinous). It is biconvex in nature.

Space between, lens and cornea is filled with a transparent watery fluid called Aqueous Humour.

The space behind the lens is filled with Vitreous Humour.

3. Retina

Image of the object is formed on retina.

Image is formed upside down on it and the process of interpretation takes place in brain.

Retina is composed of 2 types of Cells :

1. Rod Cells : Sensitive to dim light and contain the pigment Rhodopsin (in nocturnal animals more)

2. Cone Cells : Color sensitive for 3 primary colors (Red, Blue and Green).

They are found more in diurnal animals. Transferred from parents. Found more at yellow spot.

Yellow Spot (Macula Lutea) : On retina, finest image is formed here. Favea centralis is the depression in the middle of yellow spot.

Blind Spot : No image formation takes place here as the optic nerves innervate the eyeball here.

Also the rods and cones are absent here.

Color of Eye is the Color of the Iris.

Cornea is donated in Eye Donation.

Eyes glow in animals due to tapetum.

Correspondence between Camera and Eye

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Defects of Eye

1. Myopia: Cannot see distant objects, image formed before retina, concave lens is used.

The power of the lens is denoted by negative sign.

2. Hypermetropia : Cannot see near objects, image formed behind retina, convex lens is used.

The power of the lens is denoted by positive sign.

3. Astigmatism : Curvature of cornea becomes irregular and image is not clear cylindrical lens is used.

4. Cataract : Due to defective protein metabolism the lens becomes opaque Operation is needed.

5. Gloucoma : Due to defect in aqueous humour.

6. Xerophthalmia : It is due to deficiency of Vitamin A.

In this conjunctiva or cornea becomes keratinied. It may lead to blindness.

7. Presbiopia : In this, power of accommodation of lens decreases due to age factor and defected metabolism.

Also known as age sightedness. Can be removed by bifocal lens.


Divided into 3 Parts :

1. External Ear (Pinna + External Auditary Meatus)

  • Leads into auditary canal or external auditary meatus. Auditary canal terminates at ear drum or tympanum.
  • Pinna collects the sound waves and transmits them to ear drum which further transmits it to the middle ear.
  • Has wax glands which produce cerumen to trap dust particles.

2. Middle Ear (Tympanic Cavity)

It encloses 3 Ear Ossicles :

  • Malleus (Hammer – Shaped)
  • Incus (Anvil – Shaped)
  • Stapes (Stirrup – Shaped)

These 3 transmit sound to the internal ear (through stapes, which is connected to the internal ear)

Fanastra Ovalis : Connects middle ear to internal ear

Middle ear is connected to the pharynx by Eustachian tube.

3. Internal Ear (Membranous Labyrinth)

Most delicate part. Enclosed by parotic bone.

Various parts are there which perform 2 acts:

Maintenance of balance and hearing.

Succulus and cochlea parts are associated with hearing.

It is filled with a fluid, perilymph.

It ultimately leads to nerve impulses through corti (rod shaped structure), Cochlea is a small snail shaped tube, in a 2 3/4 turned spirally).

In the internal ear are 3 semi – circular canals and utriculus which are filled with a fluid, endolymph.

They perform the balancing act.
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