The skin is a natural covering and inseparable part of the human body. By separating the organism from the ex­ternal environment it performs the important function of protecting the organism from unfavourable influences of the environment. It also participates in a number of very im­portant processes, namely, thermoregulation, metabolism and excretion of waste products.

Structure ofthe Skin. The anatomical structure of the skin fits it for the performance of these important functions.

The skin is composed of three layers: 1) epidermis or external layer, 2) true skin or derma, and 3) subcutaneous adipose layer or panniculus adiposus.

The epidermis consists of epithelial cells which possess great ability to multiply and replace the destroyed cells of this layer. Owing to this ability any wounds suffered by the skin, as a result of injury or skin disease, heal quickly and without leaving a trace.

Microscopic examination of the epidermis shows that it is composed of five layers: 1) stratum germinativum or basale, 2) prickle-cell layer, 3) stratum granulosum, 4) stratum la- cidum, and 5) stratum corneum.

The stratum germinativum or basale adheres to the true skin or derma. It consists of one layer of cylindrical cells with large and easily stained nuclei. The cells of the stratum germinativum do not adhere each other, but are divided by narrow fissures. These fissures called intercellular canaliculi extend into similar canaliculi of the overlying prickle- cell layer of the epidermis. Lymph from the lymphatic fissu­res of the derma penetrates into the canaliculi of the epider­mis and circulates through them. The cells of the stratum basale are interconnected by protoplasmic bridges. The epi­dermis has no blood vessels, and the lymph entering the intercellular canaliculi brings nutrient substances into the epidermis and removes the metabolites.

The protoplasm of the cells of the stratum germinativum has grains of melanin (pigment) which are coloured from light brown to dark brown.

The cells of the epidermis multiply in the stratum germi­nativum. The young cells formed as the result of division replace the older cells and crowd them into the prickle-cell layer. No multiplication of cells is normally observed in the prickle-cell and other overlying layers of the epidermis.

The prickle-cell layer is made up of an average of 4—6 rowsof cells. The mterpapillary prominences ofthe epi­dermis have more rows of cells. The prickle-cells are large, polyhedral and have large, light nuclei. These cells also have numerous intercellular protoplasmic bridges and are separated from each other by intercellular canaliculi.

The stratum granulosum is composed of 1—3 rows of elongated cells arranged parallel to the surface of the skin. These cells have pale nuclei.

The three lower layers of the epidermis — stratum ger- minativum, prickle-cell layer and stratum granulosum — are often designated together as the malpighian layer.

Over the stratum granulosum is the stratum lacidum. Under the microscope this layer appears as a shiny thin band; it is composed of 1—2 rows of flat, shiny cells without nuclei. The protoplasm of these cells contains eleidin (an albumi­nous substance). Eleidin is a product of further transforma­tion of keratohyalin into a horny substance.

The stratum corneum is the outermost layer of the epi­dermis. It is formed of several intimately united rows of flat, thin, horny plates overlying each other. The horny pla­tes are composed of completely keratinized cells of the epi­dermis without nuclei. Their protoplasm has completely changed to keratin, the end product of the process of keratinization.

True Skin or Derma. The second layer of the skin — the true skin or derma — is located under the epidermis. The derma abounds in connective tissue fibres which form inter­weaving bundles. The connective tissue of the derma con­tains but few cells. The derma is composed of two layers: papillary and reticular. The papillary layer is next under the epidermis. The bundles of connective tissue fibres in the papillary layer are quite delicate and interweave in various directions. Many bundles run perpendicularly to the sur­face of the skin and project into the papillae.

The reticular layer consists of thicker bundles of fibres which are formed by interweaving from a dense network of connective tissue fibres. A large part of these bundles is arranged parallel to the surface of the skin. The thickness of the derma ranges in different parts of the skin from 0.5 to 4 mm. The derma has no clearly defined junction with the underlying panniculus adiposus (subcutaneous adipose layer).

The panniculus adiposus also consists of interweaving loose bundles of connective tissue fibres which form a largemesh network. The meshes of thisnetwork contain fat lo­bules-accumulations of fat cells.

The panniculus adiposus is thicker on the abdomen, hips and buttocks. It is attached to the/underlying fascia by con­nective tissue bundles.

Blood and Lymph Vessels of ще Skin. The skin has a well developed system of blood and lymph vessels. The blood vessels of the skin may hold up to one-fifth of the total mass of blood of the human organism. In the process of blood circulation the skin performs the function of an important blood depot.

The arterial trunks enter the panniculus adiposus from deeper tissues. Here they give off branches which supply the panniculus adiposus and at the junction with the derma form an arterial plexus known as the deep cutaneous vas­cular network. Another arterial plexus —the superficial cutaneous vascular network — is formed at the boundary between the reticular and papillary layers.

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