The accurate adaptation of the trochlea of the humerus, with its prominences and depressions, to the trochlear notch of the ulna, prevents any lateral movement.
2.
During elbow extension, however, the oblique posterior part makes contact with the trochlear notch on the ulna so that this obliquity forces the main axis of the forearm to form a small angle with that of the upper arm.
3.
Derived traits in catarrhini ( apes and Old World monkeys ) elbows include the loss of the entepicondylar foramen ( a hole in the distal humerus ), a non-translatory ( rotation-only ) humeroulnar joint, and a more robust ulna with a shortened trochlear notch.
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