The semilunar notch of the ulna

Ulna

The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

 (trochlear notch of ulna, greater sigmoid cavity) is a large depression, formed by the olecranon

Olecranon

The olecranon is a large, thick, curved bony eminence of the forearm that projects behind the elbow.It is situated at the upper end of the ulna, one of the two bones in the forearm...

 and the coronoid process

Coronoid process

The Coronoid process can refer to:* The coronoid process of the mandible, part of the ramus mandibulae of the mandible...

, and serving for articulation with the trochlea

Trochlea

Trochlea is a term in anatomy. It refers to a grooved structure reminiscent of a pulley's wheel.Most commonly, trochleae bear the articular surface of saddle and other joints:* Trochlea of humerus* Trochlear process of the Calcaneus...

 of the humerus

Humerus

The humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....

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About the middle of either side of this notch is an indentation, which contracts it somewhat, and indicates the junction of the olecranon and the coronoid process.

The notch is concave from above downward, and divided into a medial and a lateral portion by a smooth ridge running from the summit of the olecranon to the tip of the coronoid process.

The medial portion is the larger, and is slightly concave transversely; the lateral is convex above, slightly concave below.