The thickness of the true skin varies, according to Kolliker, from *of as line to a line and a half. In its chemical characters it agrees with those of the connective tissue, of which it is principally composed. The gelatine IL yields on boiling is derived mainly from the white fibrous tissue, and it is probably this element which is principally concerned in the changes which skin undergoes in the process of tanning. Arteries from the sub-Matteotti connective tissue freely enter into the structure of the skin, and are distributed to the fat-lobules, the sudoriparous and sebaceous (presently to be described), the hair follicles, the palmlike. etc. In these several parts they terminate in a close net work of capillaries. Those parts of the skin which border upon the epidermis are for the most part very freely provided with nerves, while in the deeper parts the nervous filaments are comparatively scanty. How they terminate is still a subject of dispute; but the view most generally adopted is that they end in loops. .
The glands occurring in the skin next claim our consideration. They are the sada•• /7paroma sweat glands, the sebaccoas or fat gla 'Ids, and the ceruminous glands. The sweat glands exkt in almost every part of the human skin. They lie in small pits in the deepest parts of the true skin, and sometimes entirely below the skin. Their orifices can be seen in the middle of the cross grooves that intersect the ridges of the papilla' on the hands and feet, their arrangement being here necessarily- regular, while in other parts they are irregularly scattered. Their size and number in different regions of the skin correspond with the amount of perspiration yielded by each part; thus they are nowhere so much developed as in the axilla or armpit. In that part of this region. which in the adult is more or Liss covered with hair, they form a layer of a reddish color, of about an eighth of an inch thick. They are soft, mid more or less flattened by their pressure on one another, being imbedded in delicate connective tissue, and covered and permeated with a net-work of capillaries. On isolating one of these ghouls. and highly nmenifying it, it is found to consist of a solitary tube, intricately raveled, one cud of which is closed, and hidden within the glandular mass, while the other emerges from the gland. The wall of the tube consists of an outer oe basement membrane, with which the blood-vessels an: in contact, and an epithelium, lining the interior. the former disappearing when the tube reaches the surface of the p•pillia. The duct, on leaving the gland, follows a meandering and rather spiral direction through the reticular portion of the ends to the Interval between the papillae, when it becomes straight; and it again assumes a spiral course in perforating the cuticle. It is not easy to exp,ain how or why so beautifully regular it spiral form should be given to the cuticular portion of the duct, which is rather wider than the rest, the average diameter of the duct being TYrf5 of en inch.
The sebaceous glands are small whitish glands. which exist in almost every part of the skin, except the palms and soles, and are especially abundant in the scalp, face (the nose being particularly rich in them), and about the anus. They are usually connected with the hairs, and consist of a duct terminating in at blind pouch-like or pear-shaped extremity. The basement membrane of these glands is lined by an epithelium, iu time particles of which are included granules of fatty or sebaceous matter, which, having become detached, constitutes the secretion. These glands are the seat of the parasite known as acarus The ceruminous glands are brown simple glands, in external appearance like the sudoriparous glands, occurring in the cartilaginous portion of the external meatus of the ear. They yield 2111 adhesive bitter secretion, which protects time membrane of the tympanum from the access of dust, insects, etc.
We shall conclude by taking a brief survey of the functions of the skin, omitting, however, its most important function, touch (q.v.). Regarded as a protective covering, the skin possesses the combined advantages of toughness, resistance, flexibility, and elasticity; the connective framework being the part which mainly these proper ties, although the epidermis co operates with it. The subcutaneous layer of fat, and the modifications of epidermis in various forms, as hairs, wool. feathers, scales, etc., serve for the preservation of warmth, and occasionally (when they occur as claws, talons, etc.) as means of offense or defense. The skin is the seat of it twofold excretion, viz., of that formed by the sudoriparous glands, and that formed by the sebaceous glands. The fluid secreted by the sudoriparous glands is usually formed so gradually that the watery portions of it escape by evaporation as soon as it reaches the surface; but in certain conditions, as during strong exercise, or when the external heat is excessive, or in certain diseases, or when the evaporation is prevented by the application of a texture impermeable tb air, as for example oiled silk, or the material known as mackiutosle'sor india-rubber cloth, the secretion, instead of evaporating, collects on the skin in the form of drops of fluid. When it is stated that the sweat contains urea, lactates, extractive matters, etc., and that the amount of watery vapor exhaled from the skin is, on an -. average, 24 lbs. daily (according to Valentin's observation), the importance of the suitor iparous glands as organs of excretion will be at once manifest. illoreover, there is reason to believe, from the experiments of Scharling, Gerlach, and others, that the importance of the skin as a respiratory organ is far from inconsiderable, very appreciable quantities of carbonic acid being exhaled hourly by the external surface of the body.