Arm and forearm
The humerus is the bone of the upper arm. Its round head fits the socket of the scapula. At the elbow its lower end meets the two bones of the forearm: the ulna on the side of the little finger and the radius on the side of the thumb.
The left humerus from the front, Gray's Anatomy, 1918, figure 207Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray207.png)
The ulna grips the end of the humerus like a wrench round a bolt, and the elbow bends and straightens on this hinge. The radius turns. When you turn the palm down, the radius crosses over the ulna while its head spins in place at the elbow. A sheet of fibres, the interosseous membrane, binds the two shafts.
The bones of the left forearm from the front, the ulna and the radius, Gray's Anatomy, 1918, figure 213Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray213.png)
The funny bone is the ulnar nerve, which runs in a groove behind the inner knob of the humerus. A knock there presses the nerve against the bone and sends a tingle to the little finger.
Sources
- Susan Standring (ed.), Gray's Anatomy, 42nd ed. (2020), chapters on the shoulder girdle and arm, the elbow and forearm.
Prev: The shoulder girdleNext: Hand and wrist