The structure of all animal hairs is very complicated, and presents many difficulties, both to the scientific observer and to those who require to manipulate these fibres in the various processes of textile and other manufactures.

As appendages of the skin they are liable, like all the epidermal growths, to great variation in structure, which is seen in its extreme form in the feathers of birds, and in the horns, hoofs, nails, and claws of the higher mammals.