The sense of smell is highly developed in dogs and can be considered a dog's primary sense. Dogs use their sense of smell for hunting, for learning where they are, for communicating with each other and for indicating food preferences. They recognize their owners and their homes more by smell than by sight. Smell is also important for locating and tasting food and even has an effect on taste: if a dog does not like the smell of his food, he will refuse to eat it.
A dog's sense of smell is a million times more sensitive than a human's and dogs have 40 times as many of the brain cells involved in deciphering odors. This high level of olfactory sensitivity can also be attributed to the olfactory mucous membrane, a receptor surface that occupies a volume of 150 cm3 (9,2 in3) in dogs, compared with only 3 cm3 in humans (0,2 in3).
The mucous membrane overlies the turbinate bones in the dog's nostrils. The turbinate bones are irregular in shape and are separated by sinuses into which the inhaled air flows, thus trapping odors. Another olfactory organ, the ethmoid bone, is also made up of sensory cells and is located at the back of the nasal cavity.
When odors come into contact with these sensory cells, they trigger chemical changes that cause a nerve signal to be sent via the olfactory nerve to the area of the brain responsible for processing olfactory information.
Perception of odors varies depending on the odor's chemical composition, the ambient humidity and the molecular weight of the scent molecules. Heavy molecules that are somewhat soluble in water are more easily smelled. These are the principles underlying the work of dogs trained to recognize human scents (rescue and tracking dogs) or objects (drug- and explosive-sniffing dogs).
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