Skunks are notorious for their anal scent glands, which they can use as a defensive weapon. They are similar to, though much more developed than, the glands found in species of the family Mustelidae. Skunks have two glands, one on each side of the anus. These glands produce the skunk's spray, which is a mixture of sulfur-containing chemicals such as thiols (traditionally called mercaptans), which have an offensive odor. A skunk's spray is powerful enough to ward off bears and other potential attackers. Muscles located next to the scent glands allow them to spray with a high degree of accuracy, as far as . The spray can also cause irritation and even temporary blindness, and is sufficiently powerful to be detected by a human nose up to 5.6 km (3.5 miles) downwind. Their chemical defense is effective, as illustrated by this extract from Charles Darwin's "Voyage of the Beagle":