Hairs, though quite generally distributed, are absent on the palms, soles, the dorsum of the last phalanges of the fingers and toes, and the lips. They vary greatly in amount, quality, and distribution, these being influenced by age, sex, climate, racial and individual peculiarities. A separation of hairs into three classes may be made: long hairs, over two inches in length,. found on the scalp, axilla, pubes, and bearded portion of the face; bristle hairs, short and stiff, as those comprising the eyebrow and eye lashes; and downy hairs of lanugo, short, fine and soft, found on parts of the face, the trunk and the extremities. Each hair may be divided into two parts; the shaft, or portion projecting above the skin surface, and the roots, or part imbedded in the skin. The shaft is made up of the cuticle, a single layer of imbricated cells covering the exterior; the cortex or main body of the hair, composed of spindle-cells, massed into elongated bundles; and the medulla or pith, of irregular cells containing air-vesicles. The color of hair is due to the presence of granular and diffuse pigment in the cortical sub stance, and to air in the cortex and the medulla. The hair-root is seated within the hair-follicle; its extremity is expanded into the hair-bulb, which is invaginated by the hair-papilla: carry ing a vascular supply. In the hair-root the cells comprising the layers mentioned are softer, and more nearly approach the character of the rete cells, from which the hair is derived. The hair-follicle is the elongated pouch-like depres sion which receives the hair-root. It is usually obliquely set, and in detail of structure is rather complex. A better understanding of its for mation may be had if it be remembered that the follicle is merely an invagination of the skin-layers; the outer portion, therefore, is made up of the derma, while the inner layers are furnished by the epidermis. Hence a dermic coat and an epidermic coat are distinguished. Each of these is further divided into layers; the dermic coat into three, namely an exter nal, fibrous and vascular; a middle, fibrous, possibly muscular in part; and an internal, thin and hyalin. The epidermic coat has several layers, the nomenclature of which is not en tirely fixed. These, too, are quite complex in structure and for practical purposes a division without description into the outer root-sheath, the inner root-sheath and the cuticle of the root-sheath will suffice. The first two are by some regarded as a part of the hair proper. These strata are derived entirely from the rete mucosum by differentiation; the corneum does not enter the follicle beyond the opening of the sebaceous gland.
The horny formations upon the distal por tions of the dorsum of the fingers and toes are strangely unlike the general skin in appearance, yet they are derived directly from the cells of the rete mucosum. For description, the nail is divided into two parts: the nail-body, or un covered portion, and the nail-root, or portion imbedded in the tissues. The entire nail rests upon the nail bed or couch. The part beneath the nail-root is known as the matrix. The de pression into which the nail is set posteriorly and laterally is the nail-groove. The skin over lying the imbedded portions is the nail-fold and the adherent film extending onto the nail-4 body from the nail-fold is termed the nail-skin. The lunula, the white curved border seen at the base of the nail represents the distal border of the matrix. The tissues. beneath the nail are
dense and vascular, but are poorly supplied with nerves. The papilla: are numerous and their arrangement in rows produces the longitudinal smite seen on the surface of the nail. The hypoderm is' devoid of fat and is closely ad herent to the periosteum of the phalanx, thus presenting a solid foundation upon which the nail may rest. Growth takes place from the matrix; although the rete mucosum underlies the entire nail, only the rete-cells of the matrix undergo the transformation into horny nail substance.
Physiology of the Sidn.— The complex his tological structure of the skin and its append ages is necessitated by physiological require ments. As the living encasement of de body its functions are necessarily related to external and internal factors. Thus it protects the or ganism from harmful influences from without; it provides a sense-organ that is necessary to a correct interpretation of the external world; it contributes largely to the regulation and con servation of body heat; and it removes from the organism quantities of waste and harmful material. All the dermal structures participate in the protective function of the skin. The connective and adipose tissue of the hypoderm; the dense fibrous and elastic meshwork of the corium; and the horny cells of the epidermis, rendered Unctuous by glandular secretion, are together admirably adapted to guard the deeper structures from injury by mechanical violence, to prevent undue loss of heat and fluids from wit.Mn, and to resist the penetrating or destruc tive action of fluids from without. The hair in man, especially on certain parts, is protective from violence and thermal change, but not to the same degree as in animals; in them it is obvious that the fur, composed of densely set hairy filaments distributed over the entire body, must act as a potent agent in turning aside harmful encroachments from the exterior, and in conserving body heat. The cutaneous nerves contribute to protection in that the sensibility of the skin may warn the individual of pos sible dangerous contacts, in time to pre vent harm.
Sensation in the skin is provided for by the presence in abundance of nervous filaments and special nerve end-organs, Although many kinds of sensations are perceived through the skin, the exact part performed by each of the several varieties of nerve terminations is not fully known as yet. By means of the tactile sense the individual is able to judge the shape of an object in contact and the portion of the skin touched by it: he can appreciate its weight and recognize whether it is hot or cold. These varieties of tactile sense—locality sense, pres sure sense and temperature sense — are not resident in one and the same variety of nerve or nerve-termination. Pain has been consid ered an overstimulation of sensory fibres; ex perimentation, however, seems to show that special nerves are concerned when pain is felt. The skin through perception of pressure and tension, is in part responsible for muscular sense, by means of which the individual is cognizant, without the aid of the eye, of the position of his body with reference to surround ings. Special sensations, as itching, burning, tickling, creeping, etc., are perceived through the skin; the nerves concerned in their reception and conduction are known chiefly as belonging to the vegetative system.