=Smell=, on the other hand, =has more sensations than we can count or name=; more sensations, probably, than all the rest of our senses put together. We can make out certain great groups of odours: flower, fruit, spicy, musky, leek, burned, rank, foul, nauseous; we may take as examples vanilla, orange, cinnamon, sandalwood, onion, toast, cheese, opium, garbage. Realise that the flower odours comprise the scents of all the flowers, as well as those of vanilla, tea, hay, and suchlike things; or that the spicy odours comprise the scents of all the spices, as well as those of thyme, geranium, bergamot, cedarwood, and suchlike things; and you will get some idea of the variety of the world of smell. When we add that odours freely blend or combine to give new scents, you will understand that the number of smell sensations is enormous. The sensory cells are found in two patches of mucous membrane, each about as big as the little-finger nail, which lie saddle-wise across the blind top of the nasal cavities. They cannot be stimulated directly; but particles carried into the outer nostrils by the breath-stream, or into the inner nostrils by the air-stream thrown back in the act of swallowing, eddy upward to them and thus arouse sensation. The second mode of stimulation plays, of course, into the hands of taste; we think we _taste_ when we swallow; we forget that we have inner nostrils, though we know very well that we can sniff up a lotion and bring it down into the back of the mouth. But though the stimulation is thus indirect, the cells are extraordinarily sensitive; a mere trace of odorous substance will set up a sensation; and the nose is also keenly discriminative.