SENSE ORGANSSense organs are specialised part of the mammalian body which

enable the animal to see, feel, smell, taste and hear. There are five sense organs, each sensitive to only certain types of stimuli and

acts independently.-The sense organs are: eye, ear, nose, eye and skin. -The stimuli received by all these organs are coordinated and interpreted by the brain and spinal cord. THE SKIN AS A SENSE ORGAN-The presence of the tactile organs in the dermis layer of the skin makes it possible for the skin to be sensitive to touch, heat, pressure, and pain.-These tactile organs are also referred to as sense organs and certain part of the skin contains more of them than the others. The following are the sensations from the skin:1. TOUCH AND PRESURE: Touch receptors are sensitive to small pressure while pressure receptors are sensitive to big pressures.

-Touch receptors are concentrated in the finger tips, face and neck. -Blind people use finger tips to decipher Braille characters, which consists of pattern of raised dots on the paper. -Meissner’s corpuscles below the epidermis are also sensitive to touch Pressure receptors or Pacinian’s corpuscles are found in deeper dermis and in joints, tendons and muscles. -Sensations of touch and pressure prevents us from injuring ourselves. -People suffering from leprosy( a disease which damages the sensory neurone in the sin) hurt themselves badly because they can’t feel touch, pressure and pains.2.HEAT AND COLD:- The sensations are detected by thermo receptors. -We have cold and heat receptors which detect cold and heat respectively.3. PAIN:-Pain receptors provides acute warning of damage to the body tissues. -Over stimulation of pressure and thermo receptors cause sensation of pain. -Special pain receptors called nociceptors are found near the surface of the skin, in the epidermis, where the tissue is most susceptible to injury. -They also are found in many internal organs. Sensitivity to pain varies throughout the body. -Surface of the eyes and skin of abdomen are more sensitive to pain than the finger tips.MAIN SENSORY RECEPTORS AND STIMULI DETECTED BY THEM1. Mechano receptors: Detects mechanical stimuli e.g; sound, touch, gravity, pressure,

pain, muscle contraction and vibration.

2. Chemo receptors: Chemical stimuli e.g; smell, taste, changes in concentration of substances(nutrients, waste products, hormones, gases ions, e.t.c in the blood. 3. Thermo receptors: Thermal stimuli e.g; cold and heat.4.Photo receptors: Electro magnetic stimuli e.g; light(intensity and colours).ORGAN OF SMELL-The olfactory receptors of the nose are found in the roof of nasal cavity with sensory cilia extending into the layer of mucus.- Impulses from the cells are transmitted to the olfactory lobe of the cortex for interpretation.N.B:1. Our sense of smell is not as acute as that of dogs.2. In some animals, smell chemo receptors are located on the skin, antennae or other body parts.ORGAN OF TASTE-Taste receptors are situated in the taste buds of the tongue’s surface. Taste buds lie in grooves on the surface of the tongue. -The taste receptors are specialized sense cells that synapse with sensory neurones. - Taste chemicals have to be in a dissolved form to stimulate the taste receptors.- We can experience four types of basic tastes: Sweet, Sour, Salty and Bitter.. All flavours are a combination of these four tastes.On the tongue, we have:1. Sweet and salty tastes at the front.2. Sour at the sides.3. Bitter at the back.ORGAN OF SIGHT: THE EYEThe organ of sight is the eye found in all vertebrates. The mammalian eye is almost spherical in

shape. The protective structures of the eye include; eye

3. Bitter at the back.

ORGAN OF SIGHT: THE EYE- The organ of sight is the eye found in all vertebrates. - The mammalian eye is almost spherical in shape. - The protective structures of the eye include; eye Socket, eyelid, eye lashes, tear glands and conjuctiva.-The eye is housed in the eye socket by six muscles which serve to hold it firmly in place and to enable it to rotate freely upward, downward and sideways without moving the head.- The eyelid does a protective job by means of blinking reflex. - It helps to moisten the eye with help of the tear gland. -The tear gland secretes tears which contains anti bacteria enzyme called lyzozyme.- - It also prevents drying up and cracking, as well as washing away dirts. -The tear gland is also called lacrimal gland. -The tear has antiseptic effect.VERTICAL SECTION OF THE HUMAN EYEThe wall of the eye ball is composed of three layers:1. Outermost sclera:- It protects the inner structure of the eye.- It bulges out in front of the eye ball to form the transparent ornea.- A thin transparent membrane, the conjuctiva is situated between the eye lid and

the cornea.

2. Middle choroid:- It contains many blood capillaries and a black pigment.- It carries nutrients to other tissues.- The pigment prevents reflection within the eye.3. Innermost retina:- It contains light sensitive cell.- The sensory cells are of two types:a. Rodsb. Cones-The rods are sensitive to different shapes of lights but not to colour.- It is sensitive to dim light.- The cones are densely packed on the fovea centralis or the yellow spot.-The most detailed visual information is sent to the brain from the cones.-Humans have 125 million rods and 7 million cones.-Retina is pigmented and well supplied with fibre and dendrites of the optic nerves.-The nerves pass through the retina at the blind spot.-It is called the blind spot because of lack of sensory cells.The Irish:-It is a pigmented ring of muscles which surround a hole called the pupil in front of the eye.- It controls the amount of light that enters the eye through the pupil.

The Lens:- It is a transparent and elastic structure.-It is biconvex in shape.-It is held by suspensory ligament to the ciliary muscles. The aqueous humour:-It is a watery fluid that fill the space between the lens and cornea while the space behind is filled with jelly-like fluid which help in maintaining the shape of the eye as well as in image formation.FUNCTIONS OF THE EYE1. Image formation:-Light enters a dense medium like the cornea, lens, aqeous humour and vitreous humour and it is mane to refract.-This causes the rays to be focused at a point to the retina.-The image formed is small and upside down, but it is corrected by the optic centre of the brain(cerebral cortex). -The rods and the cones sensory cells in the retina are stimulated and messages are relayed to the visual area of the cerebral cortex through the optic nerves.-The messages are then interpreted by the brain cells which invert the image formed on the retina and an impression of an upright object is formed.2. ACCOMODATION:-This is the ability of the eye to focus an object at varying lengths(far and near).A. ADAPTATION FOR NER VISION- The ciliary muscles contract.- The suspensory ligament relaxes(assumes normal round shape)

-The focal length shorten.B. ADAPTATION FOR DISTANT VISION-Ciliary muscles relax and are pushed outward by the pressure of the vitreous humour.-The suspensory ligaments contract due to the tension created.-The lens is stretched into an elongated shape.-The focal length is increased.DEFECTS OF THE EYE AND THEIR CORRECTIONS1. Long sightedness:- Near objects cannot be focused.- Eye can only focus on images of distant objects clearly. - while light rays from near objects are brought to a focus behind the retina.- It is also known as hypermetropia.- It is due to the eye ball being too short or the lens and the ciliary muscles having lost

their elasticity.- Loss of elasticity is mainly due to old age and is called presbyopia.- The defect can be corrected by wearing a spectacle with a suitable convex lens which

converges the light entering the eye and bring to a focus on the retina.2. Short sightedness:- Distant objects cannot be focused.- It is due to eye ball being too large or lens too powerful.- It is called myopia.

-It occurs when parallel rays of light from a distant object are brought to focus in front of the retina and in the vitreous chamber.-It can be corrected by wearing spectacles with suitable concave lens which will diverge the rays entering the eye and bring it to focus on the retina.3. Astigmatism:-Here, the surface of the cornea is uneven, i.e.; it is not smoothly curved.-Light rays entering such an eye bend unevenly at the air/ cornea boundary to produce an image that is stretched or distorted.- The light rays are not brought to sharp point on the retina.- Vertical figures can be seen clearly but horizontal ones appear blurred.- This can be corrected by wearing cylindrical lenses.4. Cataract:- The lens becomes cloudy so that sufficient light cannot pass through it.-It is common in old people.-It can be corrected by removing the affected lens and either replacing it with plastic one or providing spectacles with suitable lens.5. Night blindness:-Victims cannot see clearly in dim light.- It may be due to a deficiency of vitamin A, the substance used to make rhodopsin.6. Xerophthamia:-An eye disease caused by vitamin A deficiency.- It can lead to permanent blindness.7. Conjuctiva:- An inflamation of the conjuctiva.

-It may be caused by irritants such as wind and glare, bacteria.CARE OF THE EYE-Not rubbing them with dirty fingers.-Reading books with fairly large print and under correct lighting conditions.- Seeing a doctor in case of eye problem.

ASSIGNMENT1. Explain the dazzling of the eye when one is exposed from dark room to bright

light.2. State five differences and similarities of the human eye and the lens of a camera.3. What is binocular vision?4. Make a well labelled drawing of each of the following:a. Vertical section of the human eyeb. A section through the skinc. The tongued. The nose.