The external occipital protuberance (EOP) is a bony bump or protrusion on the back of the skull, located in the midline at the junction of the neck and the skull. It is also known as the occipital tubercle or the external occipital crest.

The EOP serves as an attachment site for several muscles, ligaments, and tendons involved in the movement and stabilization of the head and neck. The muscles that attach to the EOP include the trapezius, sternocleidomastoid, and splenius capitis muscles. The nuchal ligament, which helps to support the weight of the head, also attaches to the EOP.

In some cases, the EOP may be more prominent or larger than usual, which is known as an enlarged or prominent EOP. This is usually a benign condition that does not cause any significant health problems. However, in rare cases, an enlarged EOP may be associated with certain medical conditions or abnormalities of the skull, such as craniosynostosis or occipital plagiocephaly.

Overall, the EOP is a normal anatomical feature of the skull, and its size and prominence can vary from person to person.