ulna ul·na (ŭl'nə)
n. pl. ul·nas or ul·nae (-nē)
The larger bone of the two bones of the forearm, extending from elbow to wrist on the side opposite the thumb. Also called cubitus, elbow bone.
inner of two bones of the forearm when viewed with the palm facing forward. (The other, shorter bone of the forearm is the radius.) The upper end of the ulna presents a large C-shaped notch-the semilunar, or trochlear, notch-which articulates with the trochlea of the humerus (upper arm bone) to form the elbow joint. The projection that forms the upper border of this notch is called the olecranon process; it articulates behind the humerus in the olecranon fossa and may be felt as the point of the elbow. The projection that forms the lower border of the trochlear notch, the coronoid process, enters the coronoid fossa of the humerus when the elbow is flexed. On the outer side is the radial notch, which articulates with the head of the radius. The head of the bone is elsewhere roughened for muscle attachment. The shaft is triangular in cross section; an interosseous ridge extends its length and provides attachment for the interosseous membrane connecting the ulna and the radius. The lower end of the bone presents a small cylindrical head that articulates with the radius at the side and the wrist bones below. Also at the lower end is a styloid process, medially, that articulates with a disk between it and the cuneiform (os triquetrum) wrist bone.
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