The semispinalis capitis muscle, or musculus semispinalis capitis in latin, is the superior segment of the semispinalis muscle (‘semi’ meaning half, ‘spinalis’ meaning spine), which consists of three segments: the semispinalis capitis (‘capitis’ meaning head), the semispinalis cervicis, and the semispinalis thoracis. The semispinalis capitis muscle, as well as the other two semispinalis muscles, are part of the transversospinal system, which also includes the multifidus muscle and the rotatores muscles. This group of muscles is considered the deep layer of intrinsic back muscles, and is found deep to the erector spinae system of muscles, which includes the iliocostalis muscle, the longissimus muscle, and the spinalis muscle, as well as the more superficial splenius muscles. The transversospinal muscles all have points of origin along the transverse processes of vertebrae and mostly insert along the spinous processes of superior vertebrae. The most superficial muscle of the transversospinal system, the semispinalis capitis muscle, originates from the transverse processes of the C4 to T6 vertebrae, but unlike the other muscles in this grouping it has a robust insertion at the occipital bone, between the superior and inferior nuchal lines. This insertion can be palpated on either side of the surface of the posterior neck as a longitudinal bulge. The semispinalis capitis muscle is innervated by the posterior rami of the C4 to T6 spinal nerves, and receives its vascular supply from descending branches of the occipital artery, as well as the superior intercostal artery. The deep layers of the intrinsic back muscles all help to provide support to and control movements of the vertebral column. The semispinalis capitis muscle specifically enables extension of the head and neck when bilaterally contracted, but also works together with the semispinalis cervicis and semispinalis thoracis muscles, by way of unilateral contraction, to aid in rotation of the head and neck. Deep to the semispinalis capitis muscle lie the suboccipital and deep neck muscles, responsible for maintaining the posture of the head: the rectus capitis posterior major and minor muscles, and the obliquus capitis inferior and superior muscles.
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