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The long supinator, passing obliquely downwards and inwards, divides, in fact, the forearm into two parts: one supero-internal, the other infero-external.

The muscles of the fore-arm are, besides flexors and extensors, pronators and supinators, the former turning the hand palm downwards, the latter turning it upwards.

When the radius is broken above the insertion of the pronator teres, its upper fragment may be supinated by the biceps and supinator muscles, while the lower fragment remains in the usual semi-prone position.

Here the artery lies in the interval between the supinator longus and the pronator radii teres.

The humerus is remarkable for the great development of the supinator ridge.