The document defines and describes several key body parts and senses. It defines the brain as the mass of nerve tissue in the head, the iris as the colored round part of the eye, and muscle as tissue that contracts. It also defines neurons as special cells that send commands to muscles, the skeletal system as bones, and the skull as the bony framework of the head. Additionally, it lists smell as giving off or detecting odor, and defines taste buds as sense organs that detect taste. Finally, it lists the five main senses as sight, hearing, smell, taste, and touch.