Definition

• In order to be considered a "hand," there must be fingers and a thumb with which to grasp objects. Fingers and the thumb should fold over the palm. Many animals have "paws," but unless they have an opposing thumb it does not constitute a hand.
  
  Humans have two hands. Some members of the ape family are thought to have four as their feet have a thumb-like toe that allows them to grab and hold onto branches, food and more. Primates are the only mammals that have hands.
  
  

Anatomy

• The hand consists of a palm with four, folding fingers and one opposing thumb. Each finger has three bones with joints that allow bending. The thumb has two bones with two joints. Muscles and tendons are attached to the 27 bones.
  
  The fingers consist of the index finger, pointer finger, middle finger, ring finger, and little finger. The thumb is on the opposite side of the little finger. The hand is attached to the arm through the wrist.
  
  

How it Works

• The hand is controlled by the brain. The right hand is controlled by the left hemisphere of the brain and the left hand is controlled by the right hemisphere. The brain sends signals to the hand, telling it what it wants it to do.
  
  Two muscle groups take that message and accomplish movement in the hand. Extrinsic muscles extend from the forearm in the form of flexors and extensors. Intrinsic muscles are located inside the hand.
  
  Extensors straighten the fingers out. Flexors allow the fingers to bend and grasp. The thumb has two flexors that help with holding onto objects.
  
  The brain signals the muscles in the arm, wrist and hand to cooperate by using the extrinsic and intrinsic muscles to move the hand. With its impressive number of bones and joints, the hand can be used as a tool, or a means of communication.
  
  The hand is one of the most sophisticated aspects of the human body. With it we can operate tools, articulate ideas, or perform delicate tasks, such as surgery or needlework. The combination of muscles, tendons and bone and their direct communication with the brain allowed humans to develop.