Smell is another very important feline sense. Cats have in the region of nineteen million specialized "smelling" nerve endings in the membrane lining their noses, as compared with only five million in humans (although a long-noses dog, such as a fox terrier, has about one-hundred-and forty-seven million). On the other hand, tigers are supposed to have little or no sense of smell - which is surprising in an animal that is known for its hunting abilities.
A cat's nose is particularly sensitive to odours containing nitrogen compounds. This permits the animal to reject food that is going "off" or rancid, when it gives off chemicals rich in nitrogen.
One particular olfactory delight of cats, of course, is the plant catnip (Nepeta cataria). The reason why your cat is attracted to this garden herb, in which it may well roll and sprawl ecstatically, is that it happens to contain an essential oil which is chemically closely related to a substance excreted by a queen in her urine. As you might guess, toms are "turned on" by catnip more than queens or neutered toms. Catnip, to a cat, is very sexy vegetation! Another plant, valerian, can produce a similar response.
Many carnivores, including some cat species, make a curious, lip-curling, nose-wrinkling grimace known as "flehming". This is believed to bring some odours into contact with a little-understood organ that lies at the front of the roof of the mouth and that consists of a tiny pouch lined with receptor cells similar to the "smell" receptors of the nose.
This structure, called Jacobson's organ, seems to be involved with both smell and taste. It exists in a rudimentary and non-functional state in man but is functional in cats, although only weakly so in the domestic cat. It can be observed at its best in the mouth of a snake, where it analyzes "smell molecules" delivered by the flicking forked tongue. In cats the Jacobson's organ seems to come into play mainly in connection with odours of a sexual nature.