Skin

The skin is actually considered an organ, as it comprises several kinds of tissues that are arranged in such a manner as to function together. The primary function of the skin is to act as an impermeable (resistant) barrier between the external environment and internal tissues while assisting to hold tissues and fluids in place within the body. It also permits bodily communication with the environment through its sensory nerve distribution. The skin functions to protect the body from disease and external injury, it helps regulate body temperature and fluid retention. It synthesizes vitamin D absorption and acts as the body's primary sensory receptor to the external environment responding to heat, cold, vibration, touch, pressure and other stimuli. The skin varies in thickness from body region to region. It is thickest where there is most wear and tear, the soles of the foot for example. The skin is composed of three structural layers: the epidermis, dermis and hypodermis (some consider the hypodermis to be part of the dermis). The epidermis is the thinnest, outermost layer; it is without nerve endings or blood vessels, although sweat glands and hair follicles pass through it. The epidermis consists of five separate layers of cells. All but the deepest layers are actually dead cells that contain a protein, which toughens and waterproofs the skin. These cells form in the bottom layer of the epidermis and are pushed outward to the surface where after a period of time they are transformed into hard flakes, which form a dry outer coating of the skin. This crust flakes off and is continually being replaced with new merging cells. The epidermis is not subject to scarring from injury or exposure. The dermis or corium lies beneath the epidermis and consists of two layers: the papillary layer and the reticular layer. The outer layers join the underside of the epidermis. It consists of many capillary blood vessels, sweat glands, nerve endings and hair follicles. The deeper and thicker layer consists principally of thick interwoven bundles of elasticized fibers, which allows the skin to stretch