The semispinalis capitis is a long, thick muscle that underlies the trapezius and the splenius; but, in thin people,
it produces the rounded muscular ridge seen at the side of the median furrow
of the back of the neck. It has the same origin as the longissimus
capitis, and is inserted into the medial half of the area between the
superior and inferior nuchal lines. It is separated from its fellow muscle
of the opposite side by a membranous partition called the ligamentum nuchae.
It bends the head backwards; and is supplied by the posterior primary rami of the upper cervical nerves.