Medical Definition of splenius capitis
: a flat muscle on each side of the back of the neck and the upper thoracic region that arises from the caudal half of the ligamentum nuchae and the spinous processes of the seventh cervical and the first three or four thoracic vertebrae, that is inserted into the occipital bone and the mastoid process of the temporal bone, and that rotates the head to the side on which it is located and with the help of the muscle on the opposite side extends it
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