The hair follicle always projects somewhat obliquely into the skin, and attached to the side toward which it is leaning is a small band of non-striated muscular fibres called arrestor pili. When this acts it diminishes the obliquity of the hair and so makes it "bristle" or "stand on end," while a general contraction of these small muscles leads to the familiar condition of "gooseflesh." Nails.—The nails are specially thickened parts of the epider mis, and are divided into a root and a body. The former is con cealed by a fold of skin, and the corium on which it lies is known as the nail matrix. The body of the nail also lies on the corium, or true skin, which forms the nail bed and is very sensitive. This body of the nail is formed by the stratum germinativum and stra tum mucosum in its deeper part, and more superficially by the stratum lucidum, which is here very much thickened and con verted into keratin or horn. Near the root of each nail is a semi lunar area which is more opaque than the rest and forms the white lunula.
Glands.—Sebaceous glands are found wherever there are hairs, however rudimentary, and open by their ducts into the super ficial part of the hair follicle (see fig. 2). Their deeper or secret ing part divides into a number of bag-like alveoli composed of cells, which secrete oil droplets. There may be two or three glands to each hair follicle, and their size does not vary directly with that of the hair, since they are very large in the nose, where the hairs are quite rudimentary. They are also found on the labia minora and nipples, where no hairs are. Sudoriparous or sweat glands (see fig. 2) are found all over the surface of the body, but are specially numerous on the palms and soles. It is estimated that in the palm there are nearly 3,00o to a square inch, while in the skin of the back they do not reach 500 to the same area. In the armpits and groins they are very large. Each consists of a single long tube, lined by columnar epithelium, and coiled up into a ball or glomerulus in the subcutaneous tissue, after which it pierces the corium and epidermis to reach the surface at the pores sudoriferus. Where the stratum corneum of the epidermis is thick the duct is twisted like a corkscrew as it goes through.
The glands of Moll in the eyelids and the ceruminous or wax glands of the ear are modified sweat glands ; the former, when inflamed, cause a "sty." The skin is derived partly from the ectoderm and partly from the mesoderm of the embryo. The whole of the epidermis and its appendages are ectodermal, and in the early embryo consist of a single layer of cells; later on this becomes double, and the superficial layer, after the sixth month, is cast off and mixes with the secretion of the large sebaceous glands to form the soapy vernix caseosa with which the foetus is coated at birth. In the
meantime the cells of the deeper layer divide and form the various layers of the epidermis already enumerated. The mesodermal cells belong to the mesenchyme, and form the fibrous tissue of the true skin as well as the arrectores pilorum muscles and, in the scrotum, the dartos layers of unstriped muscle. In the sixth month fatty tissue appears in the deeper parts, and so the fat of the superficial fascia or sub-cutaneous tissue is formed. The nails are said to appear as thickenings of the epidermis at about the ninth week, quite at the tips of the digits. Later on they shift to the dorsal side, and in doing so carry the nerves in the nail bed with them. This is the only explanation available of the fact that the ventral nerves to the tips of the fingers encroach on the dorsal area. By about the twelfth week the nails are perfectly formed, but they do not reach the level of the finger tips until the eighth month. The hairs are developed in the third month of foetal life by ingrowths of the stratum mucosum of the epidermis into the corium. During the fourth and fifth months the body becomes covered by fine unpigmented hairs which are known as lanugo ; these begin to disappear about the eighth month, but some remain until after birth. On the scalp, however, the hair at birth is often more deeply pigmented than that which succeeds it. The seba ceous and sweat glands, like the hair follicles, are ingrowths of the stratum mucosum of the epidermis into the corium. The for mer become very large in the later months of embryonic life, and secrete a large part of the above-mentioned vernix caseosa. The development of the mammary gland from modified sebaceous glands has already been referred to (see MAMMARY GLAND).
For further details see J. P. M`Murrich, Development of the Human Body (London, 1923) ; J. C. Heisler, Text-book of Em bryology (London, 1907) ; Quain's Anatomy, vol. i. (London, 1908).
In the larval (gastrula) stage of the Amphioxus (lancelet) cilia are present on the surface, and in the superficial epidermal cells of some fishes and amphibian larvae there is a striated layer on the free edge which is looked upon as a relic of ancestral cilia.