Brachialis is a flexor muscle in the upper arm. It lies deep to biceps brachii, and is a more powerful flexor of the elbow.
Brachialis arises from the anterior surface of the humerus, particularly the distal half of this bone. It attaches distally at the coronoid process and the tuberosity of the ulna. The ulna does not rotate, so brachialis's only action is flexion. Pronation/supination of the forearm does not affect its action.
The base of the triangle is directed upward, and is represented by a line connecting the two epicondyles of the humerus; the sides are formed by the medial edge of the Brachioradialis and the lateral margin of the Pronator teres; the floor is formed by the Brachialis and Supinator.
It passes medialward upon the Brachialis, and piercing the medial intermuscular septum, winds around the back of the humerus between the Triceps brachii and the bone, forming, by its junction with the profunda brachii, an arch above the olecranon fossa.
As the vessel lies on the Brachialis, it gives off branches which ascend to join the superior ulnar collateral: others descend in front of the medial epicondyle, to anastomose with the anterior ulnar recurrent.
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