triangular area in the root of the neck bounded laterally by the scalenus anterior and medially by the longus colli (muscles); the two muscles meet at the triangle's apex, formed by the anterior (carotid) tubercle of the transverse process of vertebra C6; the vertebral artery arises from the subclavian artery at the base of the triangle, bisecting the triangle as it ascends to the apex to enter the transverse foramen of vertebra C6.