Occipital Region and Lamina quadrigemina

Abstract

The shape of the occipital lobe cannot be outlined from the surface of the skull. Its irregularities are compensated for by bony irregularities, so that between Lambda and Protuberantia occipitalis ext. — distance approx. 70 mm (Figs. 154 and 155) — there is fairly even external vaulting of the bone. Below this the dural relief shows the tapering bulge of the occipital pole. Over it the bone is considerably thinned out. In the midline in the vicinity of Protuberantia occipitalis ext. and int. the bone is particulary strongly developed. Asymmetries of both occipital poles and corresponding deviation of Sinus sagittalis sup. are typical asymmetries of this cerebral region. Vermis cerebelli is usually displaced contralaterally to the displacement of the occipital poles, with corresponding asymmetries of the falx, tentorium, and the large blood vessels (Fig. 156). The pole encroaches in a hooked shape into a hollow in the posterior tentorial region, and is convex on its undersurface, whereas the anterior portions of the occipital lobe base rise concavely in a forward and medial direction, corresponding to the shape of Tentorium and the slightly convex form of the underlying Lobulus quadrangularis of Cerebellum. The lateral surface shows generally an almost even curvature. The transition between the level medial surface and the base occurs gradually (Fig. 156). Fissura calcarina begins medially in the central polar region and lies in the transitional curvature between medial plane and base, rising forward. Despite all variations there is always a sharp bend forward and basally at its meeting with Sulcus cinguli. It may completely divide Isthmus gyri cinguli and end in Cisterna ambiens (in the example of Fig. 155).