The
Bloodhound, whose sense of smell is the stuff of legend, can fairly be described
as "a nose with feet." These dogs have been bred for centuries to
track game and humans. Bloodhounds are so good at what they
do that their evidence is admissible in court in most jurisdictions. If a bloodhound
tracks an escaped criminal to a specific spot, it's proof that he was there.
Despite their name, they're pretty gentle and well-mannered dogs: the "blood" comes from their original use to track animals wounded by hunters by following a blood trail.
Many other breeds, especially the Basset and the Beagle, share this ability to detect minute levels of scent and follow a trail. All of them also share certain anatomical characteristics. The immensely long nose provides for colossal numbers of olfactory cells to be spread out along it; the wide nostrils allow air to be taken in and passed over those areas effectively; and the high, domed head makes room for the extremely well-developed olfactory region of the brain. These features not only make Bloodhounds efficient, they make them comical in appearance.
The need to sniff everything is characteristic of dogs in general—dogs live in a world of scents we humans can't begin to appreciate, with our puny and ineffective noses—but the hounds carry it to extremes. A Bloodhound sometimes crosses a room by snuffling along the floor, following a scent trail only he can detect. His nose nose tells him the entire history of the floor, who passed by and when, and what brand of wax was used last.