The nuchal lines are distinct curved lines on the exterior of the occipital bone, which makes up the rear base of the skull. This trapezoid-shaped bone is interrupted with a single large hole, the foramen magnum, to allow the lower part of the brainstem, or medulla oblongata, to pass through. These lines form anatomical reference points that can be useful in examinations of the skull. They are also points of attachment for some of the muscles involved in the control of the head and neck.
One, the median nuchal line, runs down the middle of the occiptal bone. It forms a low ridge which may be understated in some people, and vanishes at the foramen magnum. The nuchal ligament attaches to the skull at the this point. In humans, this ligament does not play a major role, which is one reason the point of attachment is often minimal.
Another structure, the highest nuchal line, is located in the upper portion of the occipital bone, running perpendicular to the median. It creates a place for a structure known as the galea aponeurotica to attach. This structure is a tough membrane that underlies the scalp. Below is the superior nuchal line, which forms points of attachment for several muscles: the trapezius, occipitalis, splenius capitis, and sternocleidomastoid.
Between the superior nuchal line and the foramen magnum lies the inferior nuchal line. This last of the nuchal lines attaches to the rectus capitis posterior major and minor muscles as well as the obliquus capitis superior. Between individuals, the precise shape and size of the nuchal lines can vary. Men tend to have heavier skulls with more pronounced features, especially at locations where muscles attach. People who develop extreme strength in the muscles around the shoulders and neck, as seen with some athletes and yoga practitioners, might also have deeper nuchal lines.
This area of the skull is rarely seen in living patients, although it may be necessary to expose part of the occipital bone in surgeries. Postmortem examinations can include an evaluation if the bone was involved in the cause of death. This may be necessary for patients with severe head injuries, for instance. In the case of skeletal remains, a forensic scientist may evaluate the nuchal lines and other structures in the skull to learn more about the victim. While they cannot be used for a positive identification, they might provide clues that could help a team find out who died.