How Animals Hear动物如何听到声音
1. When we talk about ears, we usually mean the oddly wrinkled appendages on the sides of our heads.
2. We are aware that at the end of the central hole in this outer ear there is something called the middle ear, with an eardrum and a few little bones. Even deeper lies the inner ear, the organ with which we "hear".
3. Animals such as dogs and cats also have conspicuous outer ears, but few of us probably ever stopped to think whether there might be such a thing as a middle and inner ear beneath those pointed tips. Yet, we know very well that these animals hear.
4. Birds are even more mysterious, because here we do not even see an outer ear. The same is true to still a larger degree of such animals as frogs and fishes, although in the frog we can at least see an eardrum.
5. Again, at one time or another, you may have found that all such animals hear. Hunters know that birds are attracted by artificial calls, and fishermen emphasize that you should be as quiet（原文quite） as possible if you don't want to go home empty handed. And if you ever hunted frogs in your childhood, you know how softly you had to tread! Moreover, it seems absurd that birds should sing and frogs croak, if they could not even hear their own voices.
6. By direct observations and many experiments, biologists have discovered that practically all animals have some sense of hearing or vibration. Earthworms feel vibrations in the soil, fish can be trained to respond to certain tones, male mosquitoes are attracted by the sound of the female, and frogs will respond to a tape recording of their own voices.
7. The inner ear is composed of delicate membranes which bear dense patches of specialized cells called maculae. Each of these collections of cells can carry a message to the brain. What message is carried by a macula depends upon how it is affected. The message which is carried is not, however, always connected with the hearing sense. For instance, a certain kind of tadpole can tell the depth of the water it is swimming in by the pitch of a tone which is produced by its own lungs.
8. In the human and all other mammals, the macula has developed into an organ which can easily be seen. This organ is called the cochlea. This spiral shaped organ contains the macula itself and it is called "organ of Corti" after its discoverer. If you have ever seen a snail shell, you know how a cochlea looks.
9. When sound waves enter the cochlea, which is really a tube coiled around, they set a membrane into a back and forth motion and cause a new wave. This is something like the way in which high and low sounds are produced by a flute or whistle. The high sounds are produced when the air is prevented by the holes from going through, while the low sounds are produced by allowing more of the air to pass. All this is what produces the differences between high and low sounds. The loudness of a sound is evidently produced by how much the membrane is cause to move.
10. Whether or not hearing is really produced in all animals by the effect of pressure is not definitely known by scientists as yet. We do know, however, that nature has set up some very delicate hearing mechanisms for its creatures. Scientists must explore much further for more knowledge about how animals use their ears.