ANATOMY - Elbow
The elbow joint is very complex because it functions in two very distinct ways. First, it acts as a hinge to help one position their hand in space. Second, it is a rotational joint allowing for supination (turning your palm upwards) and pronation (turning your palm towards your keyboard).
There are three bones in the elbow that essentially create two separate joints to allow for these two motions. At the top of the joint is the end of the humerus (distal humerus, supracondylar humerus). The humerus has two "articulations". The first is with the ulna (olecranon process). This is the bone at the end of your elbow that you can feel. The ulna-humerus articulation allows for flexion/extension, or the hinge action of the elbow. The third bone is the radius (radial head), which is the bone on the outside of your elbow that you can feel rotating as your turn your hand facing up to facing down. This is a rotational articulation that allows for pronation/supination.
The soft tissues around the elbow are also very important. Most of the important muscles that control your hand all originate (attach) around the elbow. For example, the muscles that help you make a fist originate at the medial epicondyle, which is the bony prominence on the inside of your elbow, near your "funny bone"; it is part of the distal humerus. The muscles that help you straighten your fingers out attach on the outside of the elbow — the lateral epicondyle — which is frequently the site of tennis elbow. And don't forget the biceps and triceps muscles that attach at the elbow and allow for powerful flexion and extension movements.
There are strong ligaments that hold the elbow together. The inside ligaments — the "medial collateral" or "ulnar collateral" ligaments — are frequently injured in throwers (thrower's elbow), or in dislocations of the elbow. The outside ligaments — "lateral collateral" or "radial collateral" ligaments — can also be injured and cause certain types of elbow instability (posterolateral rotatory).
Finally, many important nerves cross the elbow. The median, ulnar, and radial nerves are the major ones, in addition to multiple other smaller sensory nerves.