There are three types of muscle tissues - smooth, skeletal, and cardiac muscle. Smooth muscle is involuntary and found in internal organs, skeletal muscle is voluntary and attached to bones, and cardiac muscle is involuntary and found only in the heart. The muscular system contracts through the sliding of the proteins actin and myosin. The integumentary system includes the skin, nails, and hair and functions to protect the body, regulate temperature, and aid excretion. The skin is composed of three layers - the epidermis, dermis, and hypodermis. Melanin and keratin are chemicals that protect skin from the sun and are found in nails and hair, respectively.