The sense of smell is used in two distinct ways, either on its own (for instance, when an object is investigated by sniffing) or in conjunction with taste, as described above. Dogs, and to a lesser extent cats, rely on their sense of smell to a much greater degree than we do, and it is perhaps surprising that olfaction is the least well understood of all their senses.

That part of the nose given over to the detection of odor volatiles is supported by a complex structure formed on the turbinate bones. Most of the epithelial surface contains no sensory cells, and is given over to cleaning, warming and humidifying the incoming air before it contacts the delicate olfactory areas. In the dog, these structures are more highly developed than in the cat, presumably because wolves are much more active than cats, and are therefore likely to draw air through their noses much more rapidly.

The olfactory epithelium, which contains the receptor neurons, is carried by the ethmoturbinal bones; some idea of the importance devoted to smell can be gained from the surface area of the epithelium, which is 18-150 cm2 in dogs (depending on breed), 21 cm2 in cats, but only 3-4 cm2 in man. The olfactory mucosa of both the dog and cat is a relatively simple structure compared to, for example, the retina (eyes). It is covered by a layer of mucus, secreted by the Bowman's glands, into which the airborne molecules which cause sensation of odor must dissolve before they can be detected.

The receptors themselves are mostly located on cilia which lie in the mucus and are attached to the receptor cells. In both dog and cat, these cilia are both longer and more numerous than in many other species, presumably to enhance either sensitivity or discriminating ability of their sense of smell. Between the receptor cells lie supporting cells, which send large numbers of microvilli into the mucus. Both receptor and support cells are continuously renewed.

Each receptor cell is a neuron, which transmits olfactory information through its axon to second-order neurons in the olfactory bulb. It has proved much more difficult to classify the receptors into types with distinct chemical specificities the way that has been possible for the neurons of taste. The distinct odor quality of particular compounds is therefore presumably generated by comparisons between the firing patterns of several different types of receptor. Because of this level of complexity (10x –12 meters evaporated to 1 billionth of a millimeter), it is difficult to make comparisons between species at the level of electrophysiology that can be interpreted in terms of olfactory function.

Cats deliberately sniff objects, and the duration of the sniffs appears to be the time that is optimum for exposure of the olfactory epithelium to the stimulus. Certainly, natural sniffing rates produce the greatest discrimination in man. Key points

Cats are unable to detect sugar, but are sensitive to sweet-tasting amino acids.
Cats lack salt-specific taste buds.