What is the smaller bone of the arm located on the thumb side?

The radius is the smaller bone of the forearm located on the thumb side. It runs parallel to the ulna, which is the larger bone located on the opposite side of the forearm. The radius is essential for many movements in the wrist and hand, particularly those involving the thumb. This bone plays a crucial role in the rotation of the forearm, allowing for the functions necessary in activities like gripping and twisting.

The phalanges refer to the bones in the fingers and toes, not specifically related to the bones of the arm. The term digit generally refers to fingers or toes as well, again not applicable for identifying the bones of the forearm. The ulna, while a significant bone in the forearm, is larger and situated on the opposite side of the radius, which means it does not fit the description provided in the question.