The horn-like substances at the end of our fingers, which we call the nails, are composed of innumerable flattened cells. These cells are generally so fused together as to be quite indistinguishable even with a microscope, but can be rendered visible by soaking a section of nail in liquor potassæ, which causes the cells to swell up and resume to a degree their original rounded form. It is worthy of remark that the animal form is built up of cells, as is the case with the vegetables, although the cells are not so variable in shape. They generally may be found to contain well-marked nuclei, two or more of the latter being often found within a single cell, and in many cases the tiny nucleoli are also visible. Good examples of these cells may be obtained from the yolk of an egg, and by careful management they may be traced throughout every part of the animal form. The teeth have many of the constituents of bone, and in some of their parts are made after precisely the same fashion. When cut, the teeth are seen to consist of a hard substance, called enamel, which coats their upper surfaces, of dentine, or ivory, within the enamel, and of “cement,” which surrounds the fangs. In Fig. 26, Plate X., which is a longitudinal section of the human “eye” tooth, is seen the ivory occupying the greater part of the tooth, coated by the enamel at the top and the cement at the bottom. In the centre of each tooth there is a cavity, which is plentifully filled with a pulpy substance by which the tooth is nourished, and which conveys the nerves which endow it with sensation. A traverse section of the same tooth is seen in Fig. 25. The enamel is made of little elongated prisms, all pointing to the centre of the tooth. When viewed transversely, their ends are of a somewhat hexagonal shape, something like an irregular honeycomb. The dentine is composed of a substance pierced with myriads of minute tubes. They require a rather high power--say 300 diameters--to show them properly. The cement is found at the root of the fangs, and is best shown in the tooth of an aged individual, when it assumes very clearly the character of bone. Sections may be made by sawing a slice in the required direction, polishing one side, and cementing it with old Canada balsam to a slide. It may then be filed down to nearly the required thinness, finished by carefully rubbing with a hone, and polished with buff leather. Canada balsam may then be dropped upon it, and a glass cover pressed firmly down. Sections of young bone form magnificent objects for the polariser. Fig. 29 is a section cut from one of the palate teeth of the ray (_Myliobátes_).