The human body has five sense organs and all perceiving various sensations. Thus, the sense organs and their stimulations received are skin, nose, tongue, eye, and ear. These sense organs have sensory cells which are specialized cells that receive particular stimulus and at the same time convert stimuli to impulses.

The following are the different senses and their functions;
The nose is used for smelling
The tongue used for tasting
The ear used for hearing
The skin used for feeling
The eyes used for seeing.


Sensory nerve endings associated with the skin are called Receptors. These receptors often heat receptors, touch receptors, and pacinian corpuscle or pressure receptors. These sensory nerve endings receive stimuli which are relayed to the brain or the spinal cord.

This is usually performed by the nose as it perceives smells of different kinds and relates it back to the brain for interpretation. The mucous membrane in the nose has tiny nerve endings that pick up odours and sends the message to the olfactory lobe of the brain where they are translated as smell.

The tongue is the organ of taste. It has small sensory organs with fine nerve branches called the taste buds which are sensitive to taste. There are only about four primary taste sensations namely; sweet, sour, salty, and bitter.

The tip of the tongue is sensitive to sweet and salty flavours. The sides of the tongue is sensitive to sour flavours, and the back of the tongue is sensitive to bitter flavours.

When these sensory cells are stimulated by substances in a solution, they transmit impulses to the brain resulting in taste solution.

The eye is the organ of sight. The wall of the eyeball is composed of three distinct layers, sclerotic, choroid, and retina.

a. The sclerotic layer is a tough and non elastic coat. It protects the delicate inner structures of the eye.

b. The choroid is the second layer. This has black pigments and a rich supply of blood capillaries.

c. The retina is elastic, very rich in blood vessels, pigmented and sensitive to light. It contains two types of sensory cells which are the rods and cones. Rods are sensitive to low light intensity while cones are sensitive to high light intensity and coloured light.

The fovea centralis or yellow spot is a part of the retina where light sensory cells are maximum and it’s therefore the most sensitive region of the retina.

The blind spot lacks the sensory cells hence it’s called the non-sensitive part of the retina. It marks the point where the optic nerves enter and leave the eye.

The iris are the free edges of the choroid layer and are pigmented and muscular.
The pupil is the hole surrounded by the iris muscle.
The lens is a transparent, biconvex crystalline structure which is kept in position by suspensory ligaments. Its shape can be altered by the contraction and relaxation of the ciliary muscles.
The ciliary muscles are attached to the outer edge of the suspensory ligaments.
The aqueous and vitreous humour are solutions of salts, sugar, and proteins in water and are transparent.

The structures that protects the eye from injury are as follows;
Eye lashes
Eye lids
Eye socket
Tear gland.


Short Sight: is an eye defect that occurs in which only near objects are seen clearly. This is because the eye lens is too long and so image of a distant object is focuses in front of the retina. It’s corrected by the use of concave lens which makes the rays entering the eye more divergent.

LONG SIGHT: This is also called Hypermetropia in which only distant object are seen clearly. This is because the eye lens is too short so that an image of a near object is focused behind the retina. It’s corrected by the use of convex lens which will converge rays of light to the correct position.

ASTIGMATISM: This happens when the curvature of the cornea is not uniform, the object is brought to a different focus in a different plane. It’s corrected by the use of a cylindrical lens.

The human eyes and the lens camera has these same similarities which are highlighted below
1. Both have converging lens
2. Both has retina
3. Both has a stop
4. The diameter of both stops is present.

The differences between the Human eye and the Lens camera are as follows
1. The lens camera has a fixed focal length while the Human eye has a variable focal length
2. The Lens camera focuses objects by moving the lens while in the Human eye, the ciliary muscles alter the focal length of the eye lens as to focus objects.


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