[21] Under the microscope, the skin is of course lighted from above by direct light from a window, or by that of a lamp concentrated by a “bullseye” condenser. The reversal of the image in the microscope often causes a pseudoscopic effect very puzzling to the beginner, prominences appearing as hollows, and _vice versa_ till the real direction of the lighting is considered. [Illustration: PLATE II. PHOTO-MICROGRAPHS OF GRAIN OF VARIOUS SKINS (_A. Seymour-Jones_). 1. Cow-hide; 2. Calf-skin; 3. East India Goat; 4. Pig-skin; 5. East India Sheep; 6. Welsh Sheep. [_Face p. 52._] [Illustration: FIG. 16.--Striped, or voluntary muscular fibre. (Ranvier.)] As stated above, the surface of skin which is next to the flesh is firmer than that in the centre, and as the fibres run nearly parallel with the surface it has a more or less membranous character. The skin is united to the body of the animal by a network of connective tissue (_panniculus adiposus_), which is frequently full of fat-cells and is then called adipose tissue. This constitutes the whitish layer which is removed, together with portions of actual flesh, in the operation of “fleshing.” If a minute portion of adipose tissue be examined microscopically, it will appear to consist of a mere mass of fat-globules entangled in connective tissue. If, however, it be stained with carmine or logwood it may be at once observed that each globule is contained in a cell, of which the nucleated protoplasm, by which the fat was secreted, is pressed closely against the wall (Fig. 15). Similar cells are contained in considerable quantities throughout the hide, and especially in the loose tissue of the central part; hence in leather manufacture it is impossible to expel or wash out the fat until the cells have been broken down by “liming” or in some other way. Many animals (ox, horse, etc.) possess a thin layer of voluntary muscle (red flesh) spread over the inner side of the skin, and used for twitching to drive off flies. In rough fleshing this is sometimes left on and may be a cause of dark flesh in sole leather. Even in the finished leather its striped structure may be detected microscopically (Fig. 16).