Rhinarium

The rhinarium (New Latin, "belonging to the nose"; plural: rhinaria) is the naked skin surface surrounding the external openings of the nostrils in most mammals. In common usage, for example among breeders of cats or dogs, the standard term is nose leather. More informally people may speak of a "wet snout" or "wet nose" because its surface commonly is moist in a healthy cat or dog.In many species there is a philtrum, a groove down the middle.

The study of the structure and of its associated functions has proved to be of considerable importance in mammalian evolution and taxonomy.

In an analogous connection of no relevance to vertebrate morphology, the term rhinarium sometimes is applied to chemosensory structures in invertebrates. For example, microscopic sensilla in the form of flattened sense organs on the antennae of aphids are referred to as rhinaria.

Morphologically the rhinarium certainly is part of the olfactory system, but it still is open to debate which part of the system it derived from. On the one hand it might be part of the main olfactory system, which captures media-borne odors; on the other, it might be part of the "second nose", the Accessory Olfactory System, which samples chemicals dissolved in fluids. An example of the former view is that the rhinarium is "an outward extension of the olfactory ... skin that covers the nasal passages, [which] contains nerve receptors for smell and touch." If that interpretation is correct, and the rhinarium extends the olfactory epithelium that lines the nasal passages, then the rhinarium is part of the main system.

In an opposing view, the philtrum ideally traces a path that continues over a notch in the upper lip, through a gap between the first incisors and premaxillae, along a "midline palatal groove" to "a canal that connects with the duct of the vomeronasal organ," part of the accessory system. Where on the one hand the moisture (mucus) may have trapped odiferous molecules in the medium, on the other hand it may be the remnant of a fluid transmission system for molecules of pheromones.

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