The medial portion of the articular surface of the humerus

Humerus

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

 is named the trochlea, and presents a deep depression between two well-marked borders; it is convex from before backward, concave from side to side, and occupies the anterior, lower, and posterior parts of the extremity. It is directly inferior to the most prominent anterior humeral fossia, the Coronoid fossia.

It articulates with 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...

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