The cells comprising the cuticle are arranged layer upon layer, and derive their nourishment entirely from the secretory vessels of the underlying derma. The cells which are immediately in contact with the true skin are more or less rounded, but become gradually altered in figure as they approach the surface, passing through the oval, the elongated oval, the elliptical, and finally being resolved into flattened scales which overlap each other much in the same way as do the tiles of a house (fig. 258). These changes in form and substance are occasioned under the combined influence of the upward pressure from below of the new crops of cells, which are constantly being developed, and of the downward pressure of the atmosphere on the surface, aided by the gradual evaporation of the fluid contents of the cells. The outermost layer of the cuticle is commonly called the horny layer, while the deeper portion, which consists of the younger and more recently-developed nucleated cells, some of them containing coloured matter (pigment cells), forms what is termed the rete mucosum. These two layers pass gradually one into the other, so that it would be difficult to say at what precise point the separation can be made. The cuticle forms a sufficiently dense covering to protect the sensitive and vascular parts beneath, and as it is constantly being thrown off from the surface in the form of scales, which are familiarly known as scurf, it follows that it must be as constantly renewed from below. It is not, however, to be supposed that the cuticle forms a perfectly continuous coat spread over the structure of the true skin. On the contrary, it is pierced by innumerable openings to afford an exit to the watery excretion of the sweat glands, the fat-like material of the sebaceous follicles, and the hairs which emerge from the openings of the hair follicles.
Fig. 258. - Microscopic Section of the Cuticle or Epidermis.
A, Horny layer. B, C, D, E, Intermediate young round cells, becoming flattened as they approach the surface. F, Derma or Cutis Vera.
Fig. 259. - 1, Simple, and 2, Compound Papillae of the Skin. A, Nerves. B, Capillary Loops. c, Tactile Corpuscles.
Although the cuticle, as has been shown, consists of different layers of cells from the deeper, recently-formed globules, through the oval, and elliptical, to the flattened scales, it constitutes a very small portion of the total thickness of the skin.