The elephant's trunk is an extension of the nose and of the upper lip. On the base of it, at the level of the bony walls which enclose the organ of smell, the two channels of the elephant's trunk are each provided with a cartilaginous and elastic valve. It is a sort of relief valve that opens and closes at will. To fill its trunk with water or to carry this water to its mouth, the elephant closes its valves after having inspired the water. To sniff a track or use its sense of smell, the valves remain open.