Dogs do not have the visual acuity that humans have. To a dog, sniffing the ground gives her as much information as we get by just looking around outside. All dogs have an innate desire to sniff everything _ places, people, other dogs. The dog's sense of smell gives it all kinds of information and is about one million times more sensitive than our own. A dog also has 40 times the number of brain cells involved in scent recognition than the number in a human.
Man makes use of this ability in dogs by training them as sniffer dogs (for finding drugs and bombs ), detectors of gas leaks, or truffle hounds.
The average dog has around 220 million scent receptors in its nose. We have around five million. The average dog has such acutely sensitive scenting ability that it can detect and identify smells that are so dilute that even the most sensitive scientific instruments cannot measure them.
Odours have a powerful influence on both the physiology and the behaviour of the dog. It's no exaggeration to say that they are genuinely mind-bending. Smell memories last for life and affect almost all canine behaviours. The dog's ability to smell the world around him and to interpret these smells depends upon a complicated chemical sensing system. The most sensitive part of the dog's nose, the septal organ, is probably responsible for initiating sniffing behaviour.
Air that is sniffed passes over a bony structure called the subethmoidal shelf, a structure that humans don't have, and on to the lining of the nasal membranes. The area above the shelf is not washed out when the dog expires air and this allows smell molecules to remain there and to accumulate. When a dog breathes in normally, the air goes through the nasal passages but continues down to the lungs, sniffing leaves air resting in the nasal chambers.
(Experiment)
In 1955, Kalmas Bjanbeh carried out experiments that showed that dogs recognize different odour secretions from one person's armpit, hand palm and sole of the foot. He also showed that dogs could distinguish between the odours of members of a single family but not between identical twins unless they were both presents.