The funny bone is actually not a bone, but refers to the ulnar nerve. The ulnar nerve extends from the hand to the shoulder, and is exposed just above the elbow, in the arm’s interior. Accidentally hitting this area can cause a significant amount of pain or tingling. In fact for some, hitting it can have people hopping about restraining curse words.
The term is also used as a scientific joke since the nerve runs across the humerus bone, which could in a spirit of levity be thought of as the humorous bone. An old saying attached to the it is that “hitting the funny bone is not humerus.” Only a small section of the ulnar nerve forms the sensitive and exposed portion. As the nerve runs along the inside of the arm, only the portion at the elbow is really the funny bone section. The rest of the nerve is better protected.
In fact, this portion is interesting from a scientific viewpoint because it is the only exposed nerve in the human body. Bone, muscles and/or ligaments protect all other nerves, so a small hit to the nerves does not generally cause pain. Unprotected, in this sense, means that this portion of the ulnar nerve is covered by skin, and perhaps a few fat cells. It is not covered or hidden among layers of deep tissue, ligaments, muscles, or bones.
Not all people react equally to a hit on the funny bone. Some people describe the incident as feeling “weird” or strange, and slightly painful. Others tend to be more sensitive and will feel a great deal of pain if they accidentally hit it on a hard object. Most people describe some degree of pain with tingling.
Thus, the funny bone is actually not very funny when struck. It is an absolute misnomer, as any person who has hit his or her elbow can attest. Unless, of course, one considers that it's “funny” in the sense of not being a bone at all.