Illustration of a dogˇs sense of smell, its receptors and the olfactory bulb which enables it to sense odorous particles in very small quantities. A dog has 250 million olfactory receptors in the nasal cavities (humans have 5 million), of which there are 1,200 different types (humans have 390) and its olfactory bulb is three times the size of a humanˇs. This gives it a much more sensitive sense of smell. It is able to detect tiny quantities of cancerous discharge in drops of sweat on the skin of cancer patients. This is a new way to detect cancer in humans.