Experiment 123.  Of course the living skin can be examined only in a general way.  Stretch and pull it, and notice that it is elastic.  Note any liver spots, white scars, moles, warts, etc.  Examine the outer skin carefully with a strong magnifying glass.  Study the papillae on the palms.  Scrape off with a sharp knife a few bits of the scarf skin, and examine them with the microscope.

236.  The Hair.  Hairs varying in size cover nearly the entire body, except a few portions, as the upper eyelids, the palms of the hands, and the soles of the feet.

The length and diameter of the hairs vary in different persons, especially in the long, soft hairs of the head and beard.  The average number of hairs upon a square inch of the scalp is about 1000, and the number upon the entire head is estimated as about 120,000.

Healthy hair is quite elastic, and may be stretched from one-fifth to one-third more than its original length.  An ordinary hair from the head will support a weight of six to seven ounces.  The hair may become strongly electrified by friction, especially when brushed vigorously in cold, dry weather.  Another peculiarity of the hair is that it readily absorbs moisture.

237.  Structure of the Hair.  The hair and the nails are structures connected with the skin, being modified forms of the epidermis.  A hair is formed by a depression, or furrow, the inner walls of which consist of the infolded outer skin.  This depression takes the form of a sac and is called the hair-follicle, in which the roots of the hair are embedded.  At the bottom of the follicle there is an upward projection of the true skin, a papilla, which contains blood-vessels and nerves.  It is covered with epidermic cells which multiply rapidly, thus accounting for the rapid growth of the hair.  Around each papilla is a bulbous expansion, the hair bulb, from which the hair begins to grow.

[Illustration:  Fig. 99.—­Epidermis of the Foot.

It will be noticed that there are only a few orifices of the sweat glands in this region. (Magnified 8 diameters.)]

The cells on the papillae are the means by which the hairs grow.  As these are pushed upwards by new ones formed beneath, they are compressed, and the shape of the follicle determines their cylindrical growth, the shaft of the hair.  So closely are these cells welded to form the cylinder, that even under a microscope the hair presents only a fibrous appearance, except in the center, where the cells are larger, forming the medulla, or pith (Fig. 106).

The medulla of the hair contains the pigment granules or coloring matter, which may be of any shade between a light yellow and an intense black.  It is this that gives the great variety in color.  Generally with old people the pigment is absent, the cells being occupied by air; hence the hair becomes gray or white.  The thin, flat scales on the surface of the hair overlap like shingles.  Connected with the hair-follicles are small bundles of muscular fibers, which run obliquely in the skin and which, on shortening, may cause the hairs to become more upright, and thus are made to “stand on end.”  The bristling back of an angry cat furnishes a familiar illustration of this muscular action.