A dog' skin is the largest and one of the most important organs of the body. It forms a barrier to protect the body of the dog from infections, parasites, and the elements. It also maintains the body's internal environment, preventing loss of moisture and other body constituents. Because the skin is on the outside of the body, it is easily exposed to outside elements and susceptible to injury and disease. It is also very visible, so disorders are readily detected during an examination. The skin is made up of layers of cells, lubricating (sebaceous) glands, blood vessels, nerve endings, and hair follicles that produce hairs. The skin cells form layers, namely the tough outer covering called the 'epidermis' and the deeper layer called the 'dermis.' The epidermis is composed of older cells that form a tough, almost impervious, protective outer barrier. As the outer cells erode, other cells mature and move up to replace them. The epidermis varies in thickness. The more exposed areas, such as the head and back, are thicker than areas such as the armpits and belly. The deeper layer (dermis) contains hair follicles, blood vessels, nerves and sebaceous (oil) glands. Hair follicles and sebaceous glands are more prevalent on the back than on the belly. Hair and nails are made of a hard substance called keratin.