Final answer:
The medial head of the triceps brachii is the most medial of the three heads of this muscle, which plays a critical role in extending the forearm.
Step-by-step explanation:
The triceps brachii is a large, thick muscle on the dorsal part of the upper arm. It often appears in the shape of a horseshoe on the posterior aspect of the arm. The primary function of the triceps is the extension of the elbow joint. The triceps brachii is a three-headed muscle that is responsible for extending the forearm. When regarding the head that is the most medial, it is referred to as the medial head of the triceps brachii. This muscle is one of the components of the posterior compartment of the arm, which includes various muscles involved in the extension and stabilization of the arm and forearm. The medial head of the triceps brachii is called the long head. It is the most medial of the three heads and is located closest to the midline of the body.