A little in front of the root of the nail the ridges of the cutis suddenly become larger and more vascular. This gives a pink hue to the nail in the greater part of its extent; while the hinder portion, separated from the front by a crescentic line, is white, in consequence of the subjacent cutis being there, more pale. You will, at once, recognise the distinction between these parts by looking at your own nails. The ridges and furrows serve, like the papillæ in other parts of the skin, to increase the surface of the cutis; and, by affording more space for the distribution of the vessels and nerves, they contribute to the sensitiveness of the part, and account for the severe pain which is caused when any foreign body is thrust under the nail. The pulp in the interior of a tooth, and the frog of a horse’s foot, are also instances in which an exquisitely sensitive structure is placed beneath a hard or horny substance. The object, in each case, is the same, viz. to give the power of taking cognizance of impressions which are made upon the surface. _Hairs._ HAIRS may also be regarded as modifications of the cuticle, because, like the nails, they are continuous with the cuticle, and are formed from the rete mucosum. Each hair (figs. 79 and 83) is received into a depression of the cutis, which is called a “follicle,” and which is lined, as far as the bottom, by cuticle (_a_), and rete mucosum (_b_). At the bottom of the follicle (_d_) the cuticle is absent, and the hair rests, directly, upon the rete; and, at this part, the rete, instead of being converted into cuticle, as it is at the sides of the follicle, becomes transformed into hair, in the following way. [Illustration: Fig. 79. Hair.] [Illustration: Fig. 80.] [Illustration: Fig. 81.] [Illustration: Fig. 82.] The cells of which the rete is composed swell out as they ascend, and so form the soft “bulb” of the hair. The outermost cells are gradually flattened, and assume an imbricated arrangement, overlapping one another like the tiles upon a roof (fig. 79, _e_, and fig. 80); and those in the interior are elongated, so as to be converted into more or less distinct fibres. The cells nearest the middle, or axis, of the hair remain moister and softer than those nearer the exterior, and form what is sometimes called the “marrow” of the hair (figs. 81 and 82).