The triceps brachii consists of three heads and is the largest muscle of the brachium. This muscle (located posterior to the humerus) functions to extend the forearm.  It takes its origin from the scapula and inserts on the ulna.  Immediately beneath the epitrochlearis is the long head of the triceps brachii.   The lateral head of the triceps brachii is lateral and posterior to the humerus.  It originates on the humerus and inserts on the ulna.  Between and beneath these two heads is the much smaller medial head of the triceps brachii.  This head originates on the humerus and inserts on the ulna.  The brachioradialis is a long, narrow band-like muscle that passes along the medial border of the forearm.  Its originates on the humerus and inserts on the  radius.