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Emitters and Receptors
A stimulus is sent from an emitter organ and received by a receptor. A stimulus can also come from the external environment, such as a change in temperature or in the hours of daylight.
There are many different kinds of emitter organs and, as you might expect, a wide variety of different receptors.
Sound: Emitter organs
There are various types of emitter organs which produce sound. Mammals have a voice box in their throats, just above the trachea. Inside the voice box there are vocal cords which vibrate as air is passed through them. The sound can vary in pitch if the vocal cords are either tightened or loosened. It is very much the same as adjusting a guitar string. If the string is tightened, the pitch is higher.
It is the shape if the lips which determines the sound which will be produced. The loudness is increased by increasing the speed of the air passing through the vocal cords.
The dolphin makes clicking noises in its throat which are amplified in its forehead. The amplified sound waves then move through the water. If they encounter something solid they are reflected back towards the dolphin. The dolphin detects the reflected sound waves using receptors near the base of its flippers and in its chin.
Male frogs and toads make croaking noises. They both have simple vocal cords; two slits in the floor of the mouth which lead to a vocal pouch. Air from the lungs is forced through the vocal cords and into the vocal pouch. This causes the vocal pouch to inflate and the sound is amplified.
Bird songs are produced by an organ called the syrinx. The syrinx is found at the point where the trachea splits into the two bronchi and is therefore quite different from the voice box or larynx of mammals. There is a membrane in the syrinx which vibrates and produces sound when air passes over it.
Animals with an exoskeleton, such as insects, make a noise by rubbing together parts of their hard, outer covering. In many cases it is the wings which are rubbed together although animals with long back legs, such as the grasshopper, are able to rub them together to produce sound.
Sound : Receptor Organs
The receptor organs which receive sound may look very different and are to be found on different parts of the animals' bodies, but essentially they function in the same way. They all include a membrane which is a thin piece of stretched 'skin'. This membrane vibrates as the sound waves in the air reach it. The vibrations of the membrane are then translated into sound by the body of the animal.
Insects have these membranes on their front pair of legs. Mammals have organs called ears and many have large structures made of skin, called pinnae (sing. pinna) which make up a part of the outer ear. These can collect sound waves and focus the sound towards the membrane and the inner ear.
Amphibians, reptiles and birds also have ears with membranes but no pinnae. It is for this reason that a bird needs to move its whole head to find the source of a sound. Fish have a lateral line on each side of the body. The lateral line picks up differences in the pressure of the surrounding water. The lateral line system, however, is thought of as an organ of touch rather than an organ of sound.