This is a disease which apparently owes its origin to a congenital defect in the constitution of the skin. The disease itself is sometimes manifest at birth, but if not it begins in the earlier years of life, gradually extends over the whole body, and continues during life. It is in a striking manner inherited (see under Inheritance).

It consists, anatomically, in an affection of the epidermis, which is sometimes greatly thickened and irregular. The thickened epidermis forms scales of larger or smaller size, sometimes producing merely a furfuraceous exfoliation, but in more severe cases giving rise to large plates like fish scales. From this latter appearance the name of the disease is derived. The scales do not consist entirely of epidermis, but contain sebaceous material, which is sometimes so abundant as to give the scales a polished appearance like mother-of-pearl. The large scales may be so fixed as to give an imbricated appearance. The milder forms of the disease, in which there is little more than an excessive scaliness of the surface, are often included under the designation Ichthyosis simplex or Xeroderma, while the more severe forms, in which there are prominent scales, are distinguished by the name Ichthyosis cornea or Ichthyosis hystrix.

In the milder forms the cutis is not markedly affected, but in some cases the papillae are greatly elongated. The substance of the corium is often atrophied, and the subcutaneous fat deficient.