Ulna
a long prismatoidal bone, at the inner side of the forearm, parallel with the radius, with which it articulates.
It is the larger and longer of the two bones, and consists of a shaft and two extremities, the upper of which forms a large part of the articulation of the elbow joint. At the upper extremity behind is a large process, the olecranon, and a smaller one, the coronoid process, in front, separated by the sigmoid or semi-lunar fossa, or olecranoid cavity, which receives the articular trochlea of the humerus. The ulna diminishes in size from above downward, and is very small at the lower extremity, which is separated from the wrist by an interarticular fibrocartilage.