Each hair grows from a tiny bulb (papilla), which is an elevation of the cutis at the bottom of a little hollow in the skin. From the surface of this bulb, the hair is produced, like the cuticle, by the constant formation of new cells at the bottom. When the hair is pulled out, this bulb, if uninjured, will produce a new one; but, when once destroyed, it will never grow again. [Footnote: Hair grows at the rate of about five to seven inches in a year. It is said to grow after death. This appearance is due to the fact that by the shrinking of the skin the part below the surface is caused to project, which is especially noticeable in the beard.] The hair has been known to whiten in a single night by fear, fright, or nervous excitement. When the color has once changed, it can not be restored. [Footnote: Hair dyes, or so-called "hair restorers," are almost invariably deleterious substances, depending for their coloring properties upon the action of lead or lunar caustic. Frequent instances of hair poisoning have occurred, owing to the common use of such dangerous articles. If the growth of the hair be impaired, the general constitution or the skin needs treatment. This is the work of a skillful physician, and not of a patent remedy. Dame Fashion has her repentant freaks as well as her ruinous follies, and it is a healthful sign that the era of universal hair dyeing has been blotted out from her present calendar, and the gray hairs of age are now honored with the highest place in "style" as well as in good sense and cleanliness.] (See p. 285.)
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.
STRUCTURE.—At the edges of the openings into the body, the skin seems to stop and give place to a tissue which is redder, more sensitive, more liable to bleed, and is moistened by a fluid, or mucus, as it is called. Really, however, the skin does not cease, but passes into a more delicate covering of the same general structure, viz., an outer, hard, bloodless, insensible layer, and an inner, soft, sanguine, nervous one. [Footnote: With a dull knife, we can scrape from the mucous membrane which lines the mouth some of the cuticle for examination under the microscope. In a similar way, we can obtain cuticle from the surface of the body for study and comparison.] Thus every part of the body is wrapped in a kind of double bag, made of tough skin on the outside, and tender mucous membrane on the inside.
CONNECTIVE TISSUE.—The cutis and the corresponding layer of the mucous membrane consist chiefly of a fibrous substance interlaced, like felt. It is called connective tissue, because it connects all the different parts of the body. It spreads from the cutis, invests muscles, bones, and cartilages, and thence passes into the mucous membrane. So thoroughly does it permeate the body, that, if the other tissues were destroyed, it would give a perfect model of every organ. [Footnote: It is curious to notice how our body is wrapped in membrane. On the outside, is the skin protecting from exterior injury, and, on the inside, is the mucous membrane reaching from the lips to the innermost air cell of the lungs. Every organ is enveloped in its membrane. Every bone has its sheath. Every socket is lined. Even the separate fibers of muscles have their covering tissue. The brain and the spinal cord are triply wrapped, while the eye is only a membranous globe filled with fluid. These membranes protect and support the organs they enfold, but, with that wise economy so characteristic of nature everywhere, they have also an important function to perform. They are the filters of the body. Through their pores pass alike the elements of growth, and the returning products of waste. On one side, bathed by the blood, they choose from it suitable food for the organ they envelop, and many of them in their tiny cells, by some mysterious process, form new products,—put the finishing touches, as it were, upon the material ere it is deposited in the body.] It can be seen in a piece of meat as a delicate substance lying between the layers of muscle, where it serves to bind together the numerous fibers of which they are composed.
Connective tissue yields gelatine on boiling, and is the part which tans when hides are manufactured into leather. It is very elastic, so that when you remove your finger after pressing upon the skin, no indentation is left. [Footnote: In dropsy, this elasticity is lost by distension, and there is a kind of "pitting," as it is called, produced by pressure.] It varies greatly in character,—from the mucous membrane, where it is soft and tender, to the ligaments and tendons which it largely composes, where it is strong and dense. [Footnote: The leather made from this tissue varies as greatly, from the tough, thick oxhide, to the soft, pliable kid and chamois skin.]