Cat's whisker. Coloured scanning electron micrograph (SEM) of a cat's whisker, or vibrissa. Whiskers are the thickened hairs that grow out of a cat's face. Like all hair they are made up of dead cells containing the protein keratin. However, unlike other hairs, their follicle is surrounded by a capsule of blood, called a blood sinus, which is itself surrounded by nerves. When a whisker is moved, some of the blood in the sinus is displaced. This greatly amplifies the message sent to the nerves and allows even small changes in air pressure to be detected. This sensitivity allows a cat to detect the movement of prey in the dark. Magnification: x2800 when printed at 10 centimetres wide.