Close behind the right sterno-clavicular articulation, but separated from it by the sternal insertions of the thin ribbon-like muscles named sterno-hyoid and thyroid, together with the cervical fascia, is situated the brachio-cephalic or innominate artery, A B, Plates 5 and 6, having at its outer side the internal jugular division of the brachio-cephalic vein, W K, Plate 5. Between these vessels lies the vagus nerve, E, Plate 6, N, Plate 5. The common carotid artery, internal jugular vein, and vagus nerve, hold in respect to each other the same relationship in the neck, as far upwards as the angle of the jaw. While we view the general lateral outline of the neck, we find that, in the same measure as the blood vessels ascend from the thorax to the skull, they recede from the fore-part of the root of the neck to the angle of the jaw, whereby a much greater interval occurs between them and the mental symphysis, or the apex of the thyroid cartilage, than happens between them and the top of the sternum, as they lie at the root of the neck. This variation as to the width of the interval between the vessels and fore-part of the neck, in these two situations, is owing to two causes, 1st, the somewhat oblique course taken by the vessels from below upwards; 2dly, the projecting development of the adult lower jaw-bone, and also of the laryngeal apparatus, which latter organ, as it grows to larger proportions in the male than in the female, will cause the interval at this place to be much greater in the one than the other. In the infant, the larynx is of such small size, as scarcely to stand out beyond the level of the vessels, viewed laterally. The internal jugular vein is for almost its entire length covered by the sterno-mastoid muscle, and by that layer of the cervical aponeurosis which lies between the vessels and the muscle. The two vessels, K C, Plate 5, with the vagus nerve, are enclosed in a common sheath of cellular membrane, which sends processes between them so as to isolate the structures in some degree from one another. The trunk of the common carotid artery is in close proximity to the vagus nerve, this latter lying at the vessel's posterior side. The internal jugular vein, which sometimes lies upon and covering the carotid, will be found in general separated from it for a little space. Opposite the os hyoides, the internal jugular vein lies closer to the common carotid than it does farther down towards the root of the neck. Opposite to the sterno-clavicular articulation, the internal jugular vein will be seen separated from the common carotid for an interval of an inch and more in width, and at this interval appears the root of the subclavian artery, B, Plates 5 and 6, giving off its primary branches, viz., the thyroid axis, D, the vertebral and internal mammary arteries, at the first part of its course.