Organs of smell and taste
Smell and taste are chemical senses: they are excited by substances dissolved in air or water. Smell receptors are in the mucous layer in the upper part of the nasal cavity; odorous substances dissolve on their hairs and excite them, and impulses go along the olfactory nerve to the brain. Smell reveals food and air quality, danger (smoke, spoiled food) and the smell of natural gas (a special odour is added to it). If one smell lasts long, the sensation weakens: this is adaptation. Taste receptors lie in taste buds of the tongue papillae; they sense sweet, sour, salty, bitter (and umami) tastes. It was once thought that each part of the tongue senses only one taste; today each taste is considered sensed by many areas of the tongue. To be tasted, a substance must dissolve in saliva; smell strongly affects taste, so when the nose is blocked food seems tasteless. Very hot or very cold food makes taste poor. Safety: do not smell or taste unknown substances; when working with chemicals, gently waft the air toward you by hand as the teacher shows. If you smell gas in the kitchen or at home, do not light a flame, do not press electric switches, open a window, leave the house with everyone and call 104 from outside. If smell or taste suddenly disappears, tell adults and see a doctor.
Safe experiment (after adult permission and checking for allergies): with eyes closed and nose pinched, taste pieces of apple and pear – see how hard it is to tell them apart with a closed nose. Never taste unknown substances, medicines or chemical products.