The skin is covered by the epider´mis or cuticle, which consists of several layers of cells. It is the epidermis which is raised and covers the bladders formed by the action of a blister applied to the skin. _Hair._--The hair consists of long solid filaments (Pl. IX. fig. 9), and not of hollow tubes, as was formerly supposed. It presents varieties of structure in different animals, which agree generally in animals belonging to the same Orders. Hairs are implanted in pits in the skin; each is swollen at the base to form the bulb, which is seated upon a papilla of the skin, by which it is formed or secreted. The hair is an epidermic formation, consisting of epidermic cells more or less flattened and altered in shape by mutual pressure. The colour of the hair is usually seated in the outer or cortical portion of the stem or shaft, and arises from the presence of aggregations of minute granules of colouring-matter or _pigment_, as the colouring-matter of animals is called: in the human hair it forms short longitudinal stripes (fig. 9). In the central pith or medullary portion of the hair the cellular structure is more open and distinct than in the cortical portion, in which the cells are so compressed and consolidated as only to exhibit the cell-structure after treatment with reagents; and the medullary cells often contain air. In grey or white hairs, the whiteness depends mainly upon the presence of air in the cells of the pith. In the gnawing or rodent animals, as the mouse or the rabbit, the pigment is partially at least situated in the cells of the medulla. In the hairs of many animals, the cuticular or surface-cells of the shaft are distinctly imbricated (fig. 5), and form beautiful microscopic objects. The principal interest in the structure of the hair relates to the three points above mentioned, viz. the position of the pigment, the arrangement of the cuticular cells or scales, and that of the cells of the pith. The pigment is best examined in hairs moistened with a little spirit of wine, which displaces the air from the cells of the pith, and renders the hair transparent; a little water should be subsequently added. The cuticular scales are also well shown by this proceeding. Towards the root of the hairs in the mouse, they project beyond the margin, giving it a toothed or dentate appearance; in the hair of the mole, the bat (fig. 5), or the wolf, this dentation may also be seen. In the hairs of some of the foreign bats, the scales are whorled, forming very beautiful objects.