One of the tubes or channels by which these glands are enabled to pour their contents to the outside of the body, and, if they be kept perfectly clean, to disperse them into the air, is seen running up the centre of the figure, and terminating in a cup-shaped orifice on the surface of the cuticle. On the palm of the hand very nearly three thousand of these ducts lie within the compass of a square inch, and more than a thousand in every square inch of the arm and other portions of the body, so that the multitude of these valuable organs may be well estimated, together with the absolute necessity for keeping the skin perfectly clean in order to enjoy full health. Fig. 1 shows a specimen of epidermis taken from the skin of a frog, exhibiting the flattened cells which constitute that structure, and the oval or slightly elongated nuclei, of which each cell has one. In Fig. 32, a portion of a bat’s wing, the arrangement of the pigment is remarkably pretty. Immediately above, at Fig. 31, is some of the pigment taken from the back of the human eye-ball. The shape of the pigment cells is well shown. Similar specimens may easily be obtained from the back of a sheep’s eye which has been hardened in spirit, or from that of a boiled fish. Fig. 33 shows the pigment in the shell of the prawn. * * * * * On various parts of animal structures, such as the lining of internal cavities, the interior of the mouth, and other similar portions of the body, the cells are developed into a special form, which is called “Epithélium,” and which corresponds to the epidermis of the exterior surface of the body. The cells which form this substance are of different shapes, according to their locality. On the tongue, for example (for which see Fig. 11), they are flattened, and exhibit their nucleus, in which the nucléolus may be discovered with a little care. Cells of this kind are rounded, as in the case just mentioned, or angular, and in either case they are termed squamous (_i.e._, scaly) epithelium. Sometimes they are like a number of cylinders, cones, or pyramids, ranged closely together, and are then called cylindrical epithelium. Sometimes the free ends of cylindrical epithelium are furnished with a number of vibrating filaments or cilia, and in this case the structure is called “ciliated” epithelium. Cylindrical epithelium may be found in the ducts of the glands which open into the intestines, as well as in the glands that secrete tears; and ciliated epithelium is seen largely in the windpipe, the interior of the nose, etc. A specimen taken from the nose is seen at Fig. 15. A beautiful example of ciliated epithelium is to be found in the gills of the mussel. A portion of one of the yellowish bands which lie along the edge of the shell on the opening side is carefully removed with sharp scissors, and examined in the shell-liquor, being protected from pressure by placing a piece of paper beneath each end of the cover-glass.