Lobus occipitalis. The anterior bound ary of the lobus occipitalis is formed in part by the sulcus occipitalis transversus, a sulcus liable to many variations respecting its position, length and direction. In addition, there are the sulci occipitales superiores and the sulci occipitales laterales. By means of these fissures the gyri occipitales superiores and the gyri occipitaes laterales are defined. Towards the oc cipital pole, the convolutions join a vertical gyms, known as the gyrus descendens (Ecker).
Insula. On penetrating the depth of the fissura cerebri lateralis, by drawing apart the edges of the bounding lobes, one comes to a deep depression, the fossa cerebri lateralis (Sylvii), at the bottom of which lies the insula, also known as the basic lobe (Stammlappen). Those parts of the lobes bounding the Sylvian fissure, which cover in the island, together constitute the operculum. Since the frontal, parietal and temporal lobes participate in its production, we distinguish a pars frontalis, a pars parietalis and a pars temporalis of the operculum. The surface of the temporal lobe directed towards the insula presents the sulci and gyri temporales transversi. Similar fissures and con
volutions exist also upon the surfaces of the parietal and frontal opercula facing the island. The insula appears in the form of an irregular conical projection, a three-sided pyramid whose apex—the island-pole—is directed forward and outward. The island is encircled by a deep fissure, the sulcus circularis (Reili). Since this furrow, strictly considered, is not circular but rather triangular, a sulcus anterior, a sulcus superior and a sulcus inferior may be distinguished. The sulcus anterior separates the island from the orbital part of the frontal operculum, the sulcus superior from the fronto-parietal operculum, and the sulcus inferior from the temporal operculum. The island is divided into a lobus insulae anterior and a lobus insulae posterior by the sulcus centralis insulae, a fissure that runs from in front and below backward and upward. The anterior lobule exhibits several short convolutions, the gyri breves . insulae, while the posterior lobule appears as the gyrus longus insulae, which now and then is subdivided into two con volutions by a long furrow which parallels the sulcus centralis insulae.