The outline of the skull, as viewed from behind, is shaped like a broad arch, convex above and on each side, and flattened below. The lambdoid suture, which has already been seen in part, can now be traced through its entire length. The serrations are deep and prominent above and behind, but become much less conspicuous as the suture is traced downwards and forwards. Inferiorly the lambdoid suture meets the ascending occipitomastoid suture and the horizontal parietomastoid suture. The posterior portions of the parietal bones, the parietal eminences and foramina, which are visible on this aspect of the skull, have already been viewed from above.
The most outstanding feature of this aspect of the skull is the external occipital protuberance (fig. 319) and the ridges which lead away from it. The protuberance is situated in the lower part of the field in the median plane and may be overhanging. It can readily be identified in the living subject, as it lies at the upper end of the median nuchal furrow at the back of the neck. The superior nuchal lines are the ridges, often sharp in character, which pass laterally from the protuberance. They form the boundary lines between the scalp and the back of the neck, and the portions of the occipital bone below them, now seen in perspective, will be examined in the norms basalis. The highest nuchal lines are curved bony ridges, which lie about 1 cm. above the superior nuchal lines. Commencing at the upper part of the protuberance they are more arched but less conspicuous than the superior nuchal lines.
In from 35 to 40 percent of skulls the maxilla and the sphenoid bone articulate with each other at the anterior end of the fissure and exclude the zygomatic bone from it.
The mastoid process and the mastoid part of the temporal bone can be seen in the inferolateral part of this aspect of the skull, but they can be examined much more satisfactorily in the norma lateralis.
Particular features.-The inion is the naive given to the most salient point on the external occipital protuberance in the median plane. The lower aspect of the protuberance gives attachment to the upper end of the ligamentum nuchae, and its posterior aspect gives origin to fibers of the trapezius muscle, which also arises from the adjoining part of the superior nuchal line. The lateral part of the superior nuchal line (Fig. 345) gives insertion to the posterior fibers of the sternomastoid and, under cover of that muscle, to fibers of the splenius capitis. The highest nuchal line gives attachment medially to the epicranial aponeurosis (galea aponeurotica) and laterally to the occipital belly of the occipitofrontalis (epicranius) muscle.
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