tactile hairs that protrude above the furry covering in mammals. The vibrissae are usually distributed in clumps on the head (on the nose, above the eyes, on the lower jaw, and so forth), and they are sometimes found on other parts of the body also (for example, in many marsupials, on the paws). They greatly resemble ordinary hairs but are several times thicker and longer. Vibrissae are specialized sense organs that perceive even the smallest variations in the environment. The base of each vibrissa is immersed in a hair follicle and surrounded by venous cavities (hence the English term for vibrissae, “sinus hairs”). Vibrissae on the head are innervated by the trigeminal nerve. The structure of the skull bones of ancient animal-like reptiles indicates that vibrissae were apparently developed in the ancestors of mammals and may be regarded as an older formation than hair. Sometimes the large hairs in the nasal cavities of humans and certain large, chitinous bristles on the bodies of insects are also called vibrissae.