This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
For purposes of protection there occur in different animals a variety of special growths from the cuticle; and those which occur in the human subject are nails and hairs.
Fig. 39. Nail ah=nd its Matrix, longitudinal section, a, Horny layer of epidermis ; b, mucous layer ; c, corium.
A Nail is simply a thickening of the outer layer of the cuticle growing from a bed or matrix, which is in the form of a fold at the back part In this fold there are two surfaces of skin looking one toward the other, and thus the root of the nail receives additions from above and below, as well as behind. It is pushed continually forwards by new growth at the bottom of the fold, and continues to receive additions to its thickness from the flat part of the matrix as long as it is adherent thereto. Hence it happens that the nail is stronger the farther from the root, and that its outer surface is harder than the deep part where the recent and soft additions to the thickness are placed.
The substance of a nail is an instance of the texture called horn. But it is always to be kept in mind that the word horn has a double meaning, which has descended to it from the Latin. This has obviously arisen from the structure of the horns of the sheep and oxen, which consist of an outer coating of the texture called horn investing a core of bone. But on the horn of the stag there is no horny covering, the structure being a growth of bone, which, when young, is covered with integument, but afterwards becomes denuded. Instances of morbid growth of solid horny texture occur occasionally in man, but rarely grow to any considerable size. In bedridden persons, whose nails have been neglected, it sometimes happens that some of them project like claws, curving over beyond the digit, and become nearly as thick as they are broad.