Medical Definition of splenius cervicis
: a flat narrow muscle on each side of the back of the neck and the upper thoracic region that arises from the spinous processes of the third to sixth thoracic vertebrae, is inserted into the transverse processes of the first two or three cervical vertebrae, and acts to rotate the head to the side on which it is located and with the help of the muscle on the opposite side to extend and arch the neck
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