The longissimus capitis is a muscle that lies under cover of the splenius immediately behind the cervical transverse processes. It is a long, slender muscle – about half the thickness of a finger. Because of that, it is easily recognized in an operation on the back of the neck, and is a guide to the depth which the surgeon has reached. It arises chiefly from the transverse processes of the upper four thoracic vertebrae and passes upwards to be inserted into the back of the mastoid temporal bone under cover of the splenius and sternomastoid.