Of the well-known "five senses"--sight, hearing, smell, taste and touch--by far the most important to most animals is smell. But compared to other mammals, humans miss out. We have evolved to rely principally on sight and we are only gradually coming to understand what other animals can achieve by smell. In mammals, smells are detected by a thin layer of very sensitive cells in the nose called the olfactory epithelium (嗅觉上皮组织). Humans only have about five square centimeters of these cells, while an Alsatian dog, for example, has 150 square centimeters. This is why dogs continue to be used by man for everything from hunting wild animals to the search for hidden drugs.