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Skin (cutis), which forms the general covering of the human body (integumentum commune), directly in contact with the external environment, performs a number of functions. It protects the body from external influences, including mechanical ones, it participates in the thermoregulation of the body and in metabolic processes, it emits sweat, sebum, performs respiratory function, contains energy reserves (subcutaneous fat). Skin occupying an area of 1.5-2.0 m 2, depending on the size of the body, is a huge field for various types of sensitivity: tactile, painful, temperature. The thickness of the skin in different parts of the body is different - from 0.5 to 5 mm. The skin is emitted by the surface layer - the epidermis formed from the ectoderm, and the deep layer - the dermis (actually the skin) of mesoderm origin.
The epidermis (epidermis) is a multilayer epithelium, the outer layer of which is gradually sloughing off. The epidermis is renewed due to its deep growth layer. The thickness of the epidermis is different. On the hips, shoulder, chest, neck and face it is thin (0,02-0,05 mm), on the palms and soles experiencing considerable physical load, 0,5-2,4 mm.
The epidermis consists of many layers of cells, united into five main layers: horny, glossy, granular, prickly and basal. The superficial horny layer consists of a large number of horny scales formed as a result of keratinization of the cells of the underlying layers. Horny flakes contain protein keratin and air bubbles. This layer is dense, elastic, does not pass water, microorganisms, etc. Horny flakes gradually slushchyvayutsya and replaced by new ones that come to the surface from the deep layers.
Under the horny layer is a shiny layer formed by 3-4 layers of flat cells that have lost nuclei. The cytoplasm of these cells is impregnated with an eleidin protein, which refracts light well. Under the shiny layer is a granular layer consisting of several layers of flattened cells. These cells contain large grains of keratogialin, which, as cells move to the surface of the epithelium, turns into keratin. In the depth of the epithelial layer there are cells of prickly and basal layers, which are united under the name of the growth layer. Among the cells of the basal layer there are pigment epitheliocytes containing pigment melanin, on the amount of which the color of the skin depends. Melanin protects the skin from the effects of ultraviolet rays. In some areas of the body, the pigmentation is particularly well pronounced (the sucking mug of the breast, the scrotum, around the anus).
The dermis, or the skin itself (dermis, s. Corium), consists of a connective tissue with some elastic fibers and smooth muscle cells. On the forearm, the thickness of the dermis does not exceed 1 mm (in women) and 1.5 mm (in men), in some places it reaches 2.5 mm (the skin of the back is in men). In the skin itself, the superficial papillary layer (stratum papillare) and the deeper mesh layer (stratum reticulare) are isolated. Papillary layer is located directly under the epidermis, consists of loose fibrous unformed connective tissue and forms protrusions - papillae, containing loops of blood and lymphatic capillaries, nerve fibers. In accordance with the location of the papillae on the surface of the epidermis, scallops of the skin (cristae cutis) are visible, and between them are oblong depressions - the furrows of the skin (sulci cutis). Scallops and grooves are best expressed on the sole and palms, where they form a complex individual pattern. It is used in forensic science and forensic science for identification (fingerprinting). In the papillary layer there are bundles of smooth muscle cells associated with the hair follicles, and in some places such bundles lie alone (facial skin, breast nipple, scrotum).
The mesh layer consists of a dense, unformed connective tissue containing bundles of collagen and elastic fibers, and a small number of reticular fibers. This layer without a sharp border turns into a subcutaneous basis, or fiber (tela subcutanea), containing in more or less the amount of fatty clumps (panniculi adiposi). The thickness of fatty deposits is not the same in all places. In the forehead and nose, the fat layer is weakly expressed, and on the eyelids and skin of the scrotum it is absent. On the buttocks and soles the fat layer is especially well developed. Here it performs a mechanical function, being an elastic litter. In women, the fat layer is better developed than in men. The degree of fat deposition depends on the type of build, fatness. Fatty deposits (fatty tissue) are a good thermal insulator.
The color of the skin depends on the presence of pigment, which is found in the cells of the basal layer of the epidermis, and also occurs in the dermis.
Vessels and nerves of the skin
The skin penetrates the branches from the superficial (cutaneous) and muscular arteries, which in the thickness of the skin form a deep dermal and superficial podsoskovuyu arterial networks. A deep skin network is located on the border of the skin itself and the subcutaneous fat base. The thin arteries leaving from it branch and blood supply the fatty segments, the skin itself (dermis), sweat glands, hair, and also form the arterial network at the base of the papillae.
From this network, the blood supply of the papillae is carried out, into which the capillaries, which form the intracapillary capillary loops, reach the tips of the papillae. From the surface network, thin vessels emerge to the sebaceous glands and hair roots. Venous blood from the capillaries flows into the veins, forming a superficial podsosochkovoe, and then a deep podosochkovoe venous plexus. From the deep podsozochkovogo plexus venous blood flows into the deep dermal venous plexus, and then into the subcutaneous venous plexus.
Lymphatic capillaries of the skin form a superficial network in the reticular layer of the dermis, into which the capillaries, which lie in the papillae, flow into the deep, near the border with the subcutaneous fatty tissue. Formed from the deep network of lymphatic vessels, connecting with the vessels of the fascia of the muscles, are directed to the regional lymph nodes.
Innervation of the skin is carried out as branches of somatic sensory nerves (cranial, spinal), and fibers of the autonomic nervous system. In the epidermis, papillate and reticular layers there are numerous, different in structure nerve endings, perceiving touch (touch), pressure, pain, temperature feeling (cold, heat). Nerve endings in the skin are distributed unevenly. They are especially numerous in the skin of the face, palms and fingers of the hand, external genitalia. The innervation of the glands, muscles lifting the hair, blood and lymph vessels is carried out by postganglionic sympathetic fibers entering the skin as part of the somatic nerves, and also along with the blood vessels. Nerve fibers form plexuses in the subcutaneous adipose tissue and in the papillary layer of the dermis, as well as around the glands and roots of the hair.