part of longissimus longissimus capitis A long slender muscle between the longissimus cervicis and the semispinalis capitis that arises by tendons from the upper thoracic and lower cervical vertebrae, is inserted into the posterior margin of the mastoid process, and extends the head and bends and rotates it to one side. BTO:0001649 BrendaTissueOBO musculus longissimus capitis trachelomastoid muscle