Which of the following bones is not part of the ossicular chain in the middle ear?

The ossicular chain in the middle ear is composed of three small bones known as the malleus, incus, and stapes. These bones play a crucial role in the process of hearing by transmitting sound vibrations from the eardrum to the inner ear, where they are converted into neural signals.

The malleus, commonly referred to as the hammer, is attached to the eardrum. The incus, or anvil, is situated between the malleus and stapes. Lastly, the stapes, known as the stirrup due to its shape, connects to the inner ear. Together, they enhance the efficiency of sound transmission.

In this context, the term "helmet" does not refer to any known bone in the human anatomy, especially not in relation to the ossicular chain. Therefore, the selection of "helmet" highlights a term that is not associated with the anatomy of the ear.