Biology, SS 3 Week: 8
Topic: Sense Organs
Introduction
A sense organ is defined as a group of specialized cells, tissues or receptors which are able to receive, perceive or detect stimulus and transmit the information, impulse or message to the central nervous system.
There are five sense organs in human body. These sense organs are eye, ear, nose, tongue and skin.
The Skin
The skin is the outer layer of vertebrate animals. Its major functions are protection, temperature regulation and to act as a sense organ.
There are two major layers of the skin: epidermis and dermis.
Epidermis
The epidermis is the outer renewable layer of the skin. As it wears away at the surface it grows at the base.
Malpighian Layer: This is the base layer, which is constantly producing new cells by mitosis. The new cells are pushed towards the surface. The dark pigment melanin is produced here.
Granular Layer: The protein keratin accumulates in the cells giving them a granular appearance. The cells finally die.
Cornified Layer: This is the surface layer of dead keratinized cells, which is constantly being eroded.
Dermis
The dermis is the inner layer of the skin, above it is the epidermis and below it is adipose tissue.
Many different structures are present in the dermis: hair follicles, sebaceous glands, sweat glands, blood vessels, sensory nerve endings all of which are embedded in a dense matrix of connective tissue.
Functions of the Skin
- Prevents excessive loss of water — the cornified layer of the epidermis is waterproof.
- Prevents the entry of pathogens — the cornified layer is made of dead cells.
- Sebum from the sebaceous glands contains anti-microbial chemicals.
- Sebum oil’ keeps the skin intact preventing it from ‘cracking’.
- Melanin gives protection against the damaging UV rays of sunlight.
- The dermis and adipose tissue protect against mechanical injury.
- Temperature Regulation – maintains blood at 37°C
The Skin as a Sense Organ
The skin contains receptors for touch, pressure, pain, temperature rise and temperature decrease. The skin supplies information about a variety of external environment condition. The sensory nerve endings are unevenly located within the skin. Those that are sensitive to pressure (pacinia corpuscles) are found deepest in the skin and therefore require a stronger stimulation to register in the brain. Those that are sensitive to touch (meissner’s corpuscles) are largely distributed and they are closest to the surface of the skin especially in areas without hairs like tongue, lips, forehead, fingers, etc. They need a mild and gentle stimulus to register in the brain. Found between the receptors of pressure and touch is the ones which detect cold, heat and pain.
The Eye (The Organ of Sight)
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, eyelids, eye lashes, tear glands, and conjunctiva. The eye is housed in a protective cavity in the skull called the eye socket. The eye is attached to the socket by six muscles which serve to hold it firmly in place and enable it rotate free upward, downward and sideways without moving the head.
Structure of the Eye
- Sclera or Sclerotic layer: tough outer white layer of the wall of the eye. It gives shape and firmness to the eye.
- Cornea: transparent ‘window’ of the eye, focusing of light on the retina.
- Iris: the coloured sheet of muscle, controls the pupil size so controls entry of light.
- Pupil: a hole in the iris letting light into the back of the eye.
- Ciliary Body: a ring of muscle controlling the shape of the lens.
- Suspensory Ligaments: transfer the pull of the ciliary body to the lens.
- Lens: accommodation — the fine adjustment to the focusing of light onto the retina.
- Retina: light sensitive layer of rods and cones converting light into nerve impulses.
- Fovea or Yellow Spot: a tiny area of densely packed cones for detailed and coloured vision.
- Choroid: a black-pigmented layer preventing internal reflection of light.
- Blind Spot: exit point of the optic nerve cutting through the retina so no rods or cones here.
- Optic Nerve: carries the impulses from the rods and cones to the visual centre of the brain.
- Aqueous Humour: a clear liquid in front of the lens maintaining the shape of the cornea.
- Vitreous Humour: a clear jelly offering support and shape to the back of the eye.
Functions of the Eyes
The eyes perform two major functions. These are: image formation and accommodation.
Image Formation: The ability to see involves many processes. Light enters a denser medium like the cornea and lens and is made to refract. This causes the rays to be focused at a point to the retina. The image formed is upside down, but is corrected by the optic centre of the brain (cerebral cortex).
Accommodation: Accommodation is the ability of the eye to focus an object at varying lengths. In other words, accommodation is the ability to see clearly (far and near) objects.
Adaptation for rear vision:
- The ciliary muscles contract.
- The suspensory ligament relaxes.
- The lens also relaxes (assumes normal round shape).
- The focal length shortens.
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.
Eye Defects and Correction
- Short Sight (myopia):Condition: can view close object clearly but distant objects are out of focus. Light rays are focused short of the retina. It is caused as a result of the eyeball being too long or the focusing elements of the eye are too strong.
Correction: use a concave (divergent) lens to widen the angle over which the light rays have to be refracted.
- Long Sight (hypermetropia): can view distant object clearly but close objects are out of focus, the focal point is long of the retina i.e. is behind retina. It is caused as a result of eyeball being too short or the focusing elements of the eye are too weak.
Correction: Use a convex (convergent) lens to reduce the angle over which the light rays have to be refracted.
- Astigmatism: It is caused by irregularities in the curvature of the cornea and the lens leading to the blurring of image.
Correction: the use of cylindrical lenses.
- Night Blindness: Night blindness is a deficiency disease caused by lack of vitamin A.
Correction: Use of vitamin A.
Care of the Eyes
- Bath with salt solution
- Use a clean handkerchief for cleaning the eyes.
- Consult a medical doctor in case of suspected infection
- Use antibiotic eye drops e.g. chloramphenicol and visine in case it is inflammed (or turns reddish).
