This vessel, as has been stated, arises from the arteria innominata immediately behind the upper part of the sterno-clavicular articulation, and inclines a little backwards as it ascends in the neck. In this stage it is covered anteriorly by the integuments, by the platysma myoides (except in the immediate neighborhood of its origin from the innominata);—more deeply by the sterno-mastoid enclosed within a proper sheath of the cervical fascia; by the sterno-hyoid and sterno-thyroid muscles, and still deeper, by branches of the descendens noni nerve, and by the cervical fascia. When the sterno-mastoid muscle is largely developed, its sternal portion considerably overlaps the artery after its origin from the arteria innominata. Internally it is related to the trachea, oesophagus, and thyroid gland, which often overlaps it; and to the larynx and inferior portion of the pharynx. Externally it is related to the internal jugular vein and pneumogastric nerve, which latter lies deeply concealed between the artery and vein,—the nerve, artery, and vein being contained in a common sheath of fascia: sometimes a distinct septum of the same structure passes from the front to the back part of the sheath, so as to separate the artery from the vein. Posteriorly, it is crossed by the inferior thyroid artery, which separates it from the vertebral : the sympathetic nerve and its branches descending, and the recurrent nerve ascending, and some loose areolar tissue lie also behind the common carotid, and separate it from the spine and longus colli muscle.