Wherever hair exists, tiny muscles are found, interlaced among the fibers of the skin. These, when contracting under the influence of cold or electricity, pucker up the skin, and cause the hair to stand on end. [Footnote: In horses and other animals which are able to shake the whole skin, this muscular tissue is much more fully developed than in man.] The hairs themselves are destitute of feeling. Nerves, however, are found in the hollows in which the hair is rooted, and so one feels pain when it is pulled. [Footnote: These nerves are especially abundant in the whiskers of the cat, which are used as feelers.] Thus the insensible hairs become wonderfully delicate instruments to convey an impression of even the slightest touch. FIG. 24. [Illustration: A, _a perspiratory tube with its gland;_ B, _a hair with a muscle and two oil glands;_ C, _cuticle;_ D, _the papillæ;_ and E, _fat cells._] Next to the teeth and bones, the hair is the least destructible part of the body, and its color is often preserved for many years after the other portions have gone to decay. [Footnote: Fine downy hairs, such as are general upon the body, have been detected in the little fragments of skin found beneath the heads of the nails by which, centuries ago, certain robbers were fastened to the church doors, as a punishment for their sacrilege.] THE NAILS protect the ends of the tender finger, and toe, and give us power more firmly to grasp and easily to pick up any object we may desire. They enable us to perform a hundred little, mechanical acts which else were impossible. At the same time, their delicate color and beautiful outline give a finish of ornament to that exquisite instrument, the hand. The nail is firmly set in a groove (matrix) in the cuticle, from which it grows at the root in length [Footnote: By making a little mark on the nail near the root we can see, week by week, how rapidly this process goes on, and so form some idea of what a multitude of cells must be transformed into the horny matter of the nail.] and from beneath in thickness. So long as the matrix at the root is uninjured, the nail will be replaced after any accident. (See p. 288.) III. THE MUCOUS MEMBRANE.