Joemella Rinon BEED-1B Soc Sci 1

The External Senses

VISION

The eye is the sense of organ for vision. It is sense for sight and arranged like a camera to focus on light reflected from or generated by objects outside the body onto a sheet of receptor cells, the retina at the back of the eye.

Vision depends on the interaction of the eyes and the brain.

Structure of the Eye The adult is a globe-shaped and has a diameter of approximately

1 inch.

It composed of 3 coats:

Sclera (the outer coat) – is a tough opaque layer of connective tissue used to protect the inner structure of the eye. It also help the to maintain the shape of the eyeball.

Choroid Coat (the middle coat) – is the pigmented layer.it contains some blood vessels that supply the eye with blood. It also absorbs imperfectly focused light rays.Retina (the inner coat) – it contains approximately 115 000 000 rods and 6 500 000 cones. These are light sensitive cells and are the receptors in the eye. Fovea is the most sensitive part of the retina which contains only cones tightly packed together. Blind

Spot the region of the retina where there are no rods and cones. Defects of Vision

1. Rods are shaped like cylinder the function of it is chiefly under condition of low illumination and send information to the about movement and about white, gray, and black but about color.

2. Cones their name implies are conical shape the function of it is bright light and provide information not only about movement and about the black and white dimension but also the color.1. Presbyopia – is a special form of farsightedness

which occurs with advancing age. 2. Farsightedness or Hyperopia – is caused by the

shortened eyeball, making the distance from the lens to the retina too short.

3. Nearsightedness or Myopia – if the eyeball is elongated the distance from the lens to the retina is too long.

4. Astigmatism - it is a structural defect cause of the eye generally caused by an irregularity of in the shape of the cornea.

5. Colorblindness – poor color vision can be caused by an inherited lack of one or another of the 3 types of cones.

HEARING The sense of hearing in many ways the human being’s most

vital channel of interaction with the environment. The stimulus for hearing comes in the form of sound waves

produced by vibrations or movements of the sound source. The human ear can register vibrations of air particles ranging in the frequency from 16 to 20 000 per second.

3 dimensions that describe the sound stimulus are its intensity, its frequency, and its complexity. This dimension gives rise to the sensory dimensions of loudness, pitch, and

timbre.

Structure of the Ear

The ear is divided into 3 parts: the outer, the middle, and the inner ear. Each of these parts serves a vital function

in the transforming pressure in the air into auditory sensations.

The outer ear, visible part of the ear is composed of the pinna, the auditory canal, and the tympanic membrane.

The middle ear, is an air-filled chamber that is connected to the pharynx, by the Eustachian tube. The middle ear structure is composed of 3 small bones or ossicles : malleus (hammer), the incus (anvil), and the stapes (stirrup). The inner ear is a rather complicated chambers and canals

called collectively the series of the labyrinth. One finds the cochlea which is a fluid-filled bony structure shaped like a

snail shell. There 3 canals in cochlea – the cochlear canal, the tympanic canal, and the vestibular canal.

Hearing Defects1. Conduction deafness – this can be produced by some

defect in the system of bones and membrane that conduct sound stimulus to the inner ear. Deafness due inability to transmit vibrations though the external and middle ear.

2. Nerve deafness – this kind of deafness results from damage to the nerves themselves or to the delicate parts of the cochlea.

SMELL At the very top of the nasal passages lies the olfactory

epithelium, the membrane that contains the receptors sensitive to the smell. In man, there are about 5 square millimeters of olfactory epithelium in each nasal cavity. The receptors for smell called olfactory rods. The receptors cells are individual cells scattered between supporting cells. Each receptor cell is a modified nerve cell with elongated dendrite terminating in hairlike processes at the surface of the nasal epithelium. They are sensitive only to gases to volatile substances that have been dissolved in the air. As we breathe normally, the air flows from the nostrils to the throat taking a direct route, but a certain amount rises gently to touch the olfactory epithelium. Sniffing helps in bringing stimuli to these sensitive areas. When the receptors are stimulated, they send neural impulses to the brain by way of the olfactory nerves. The olfactory sense adapts to a constantly presented stimulus. The adaptation to a constant smell is quite complete after a few minutes. The olfactory sense also responds to a reduction of olfactory stimulation with an increased sensitivity.

TASTE What is generally called the “taste” of the food depends only

in the small part to the sensory receptors for taste. The receptor cells for the taste are located in taste buds on the upper surface of the tongue and to a lesser extent, on the surface of the pharynx and larynx. The receptor cells are specialized cells with hairlike processes on their outer end.

These receptors respond to four basic tastes: sweet, salty, sour, and bitter. Some of the taste buds respond only one of them, and some to all four. Receptors for the sweetness are found at the tip of the tongue, for sourness at the side, for saltiness at the tip and the side, and for bitterness at the base of the tongue.

Food in solution spreads over the tongue, enters the taste buds and sets off chemical changes that stimulate the receptors to set off neural impulses that are sent to the brain

SKIN SENSES The skin has four separate sense pressure, coldness, and warmth. The nerve ending in the skin come in four general

forms - some in the little branches called free nerve ending,

some in globular bulbs, some in egg-shaped corpuscles, and some in the form of “basket” surrounding root hairs.

The sense of the pain is not a simple one. It arise from extreme stimulation of many different kinds of stimuli a wide variety of bodily areas. The actual stimulus to pain is probably the destruction of nerve tissues.

The sensation of cold appears to be simply the stimulation of cold receptors. The cold receptors are responsive to temperature which is cooler than the body and the warmth receptors respond to stimuli that are warmer than the body. The simultaneous excitation of the warm and cool sense receptors yields a sensation of hotness called paradoxical heat.