Lack of resistance to injury, incapacity for adaptation, whether it be due to a congenital defect or to an acquired condition, is not in itself a disease, but the disease is produced by the action on such an individual of external conditions which may be nothing more than those to which the individuals of the species are constantly subject and which produce no harm.
[Illustration: FIG.

3 .-- A SECTION OF THE SKIN.1.A hair.

Notice there is a deep depression of the surface to form a small bulb from which the hair grows.2.The superficial or horny layer of the skin; the cells here are joined to form a dense, smooth, compact layer impervious to moisture.3.The lower layer of cells.

In this layer new cells are continually being formed to supply those which as thin scales are cast off from the surface.4.Section of a small vein.

9.
Section of an artery.8.Section of a lymphatic.