Skin

"The skin, like a cloak, covers us all over, the oldest and the most sensitive of our organs, our first medium of communication, and our most efficient of protectors. the whole body is covered by skin. Even the transparent cornea of the eye is overlain by a thin layer of modified skin. the skin also turns inwards to line orifices such as the mouth, nostrils, and anal canal. In the evolution of the senses the sense of touch was undoubtedly the first to come into being. Touch is the parent of our eyes, ears, nose, and mouth. It is the sense which became differentiated into the others, a fast that seems to be recognized in the age-old evaluation of touch as 'the mother of the senses'."

Montagu, Ashley, 1978: Touching-the Human Significance of Skin,
2nd ed., Harper & Row, ISBN 0-060-90630-8