Digestion begins in the mouth - saliva lubricates food and adds starch-digesting enzymes to it, the teeth crush and chew it, and the muscular tongue pushes it around the mouth, then rolls it into a ball for swallowing. As the tongue pushes the ball past the soft palate, the epiglottis shuts like a lid over the entrance to the windpipe, and the ball slides into the oesophagus.
Saliva is 99.5 per cent water and is continually produced to clean the mouth - as well as digestive enzymes it contains an enzyme which destroys bacteria. Food entering the mouth stimulates taste buds on the tongue; the brain receives their messages and instructs the salivary glands to go into top gear. There are three pairs of saliva glands, the parotids just in front of the ears, which open at the back of the mouth near the molars, and the submandibulars and submaxillaries, which open beneath the tongue. Each pair of glands produces a slightly different mix of mucus and enzymes. Saliva is copiously produced in response to acids - vinegar, lemon juice, etc. Nausea also produces reflex salivation.
Some of the sensations we call 'tastes' are in fact smells. As we chew, the volatile constituents in our food are wafted up into the nasal cavity where they are sampled by the smell receptors. Altogether the tongue has about 2000 taste receptors or taste buds on its sides and upper surface. These detect four basic tastes: salt, sweet, sour, and bitter. All taste sensations - whether we interpret them as cheese, chocolate, tea, or apple pie - are blends of these primary tastes. Sweet things are tasted at the front of the tongue, salt and sour at the sides, and bitter towards the back. There are also a few taste buds on the soft palate and in the throat.
The vocal cords lie in the larynx (voice box), the short passageway between the throat (pharynx) and trachea (windpipe). When we decide to talk, muscles in the glottis contract, pulling the vocal cords (tiny elastic ligaments) taut; the outrush of air, speeded up because the glottis is narrowed, causes the cords to vibrate. The more forcefully air rushes past the cords, the louder the noise we make; the greater the tension in the cords, the higher our voice sounds. Intense emotions, infections, dry air, and irritants such as cigarette smoke can all prevent the cords from vibrating freely. |
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