Plate 4
COMMENTARY ON PLATES 5 & 6.
THE SURGICAL FORM OF THE DEEP CERVICAL AND FACIAL REGIONS, AND THE RELATIVE POSITION OF THE PRINCIPAL BLOODVESSELS AND NERVES, &c.
While the human cervix is still extended in surgical position, its deeper anatomical relations, viewed as a whole, preserve the quadrilateral form. But as it is necessary to remove the sterno-cleido-mastoid muscle, in order to expose the entire range of the greater bloodvessels and nerves, so the diagonal which that muscle forms, as seen in Plates 3 and 4, disappears, and thus both the cervical triangles are thrown into one common region. Although, however, the sterno-mastoid muscle be removed, as seen in Plate 5, still the great bloodvessels and nerves themselves will be observed to divide the cervical square diagonally, as they ascend the neck from the sterno-clavicular articulation to the ear.
The diagonal of every square figure is the junction line of the opposite triangles which form the square. The cervical square being indicated as that space which lies within the mastoid process and the top of the sternum—the symphysis of the lower maxilla and the top of the shoulder, it will be seen, in Plate 5, that the line which the common carotid and internal jugular vein occupy in the neck, is the diagonal; and hence the junction line of the two surgical triangles.
The general course of the common carotid artery and internal jugular vein is, therefore, obliquely backwards and upwards through the diagonal of the cervical square, and passing, as it were, from the point of one angle of the square to that of the opposite—viz., from the sterno-clavicular junction to the masto-maxillary space; and, taking the anterior triangle of the cervical square to be that space included within the points marked H 8 A, Plate 5, it will be seen that the common carotid artery ranges along the posterior side of this anterior triangle. Again: taking the points 5 Z Y to mark the posterior triangle of the cervical square, so will it be seen that the internal jugular vein and the common carotid artery, with the vagus nerve between them, range the anterior side of this posterior triangle, while the subclavian artery, Q, passes through the centre of the inferior side of the posterior triangle, that is, under the middle of the shaft of the clavicle.
The main blood vessels (apparently according to original design) will be found always to occupy the centre of the animal fabric, and to seek deep-seated protection under cover of the osseous skeleton. The vertebrae of the neck, like those of the back and loins, support the principal vessels. Even in the limbs the large bloodvessels range alongside the protective shafts of the bones. The skeletal points are therefore the safest guides to the precise localities of the bloodvessels, and such points are always within the easy recognition of touch and sight.
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.