Known as the largest organ of the human body, skin is used for insulation, production of vitamin D, sensation, and excretion through sweat. Skin of mammals often contains hairs, which function to enhance the insulation provided by the skin. Comprised of the epidermis and dermis, the skin protects the muscles and organs of the body. The hypodermis can be found beneath the dermis, however it is not considered part of the skin, as it is used only to connect the skin to bones and muscles and supply it with blood vessels and nerves. The epidermis is the outermost layer and does not contain blood vessels. It can be divided into 5 strata: corneum, lucidum, granulosum, spinosum and basale. Below the epidermis lies the dermis, which contains blood vessels, nerves, hair follicles, smooth muscle, glands and lymphatic tissues. It is comprised of connective tissue of collagen, elastin and reticular fibres. The dermis can be subdivided into the papillary and reticular layers. The papillary layer is the outermost layer and supplies the dermis with blood vessels, while the reticular layer is connected with the hypodermis. Generally skin can be classified as thick or thin. Thick skin is that which is located on the soles of the feet and palms of the hands, and serves to help grip. Thin skin occurs everywhere else on the body and is softer and more flexible.