The angular gyrus is behind the supramarginal gyrus and between the first and second intermediate rami of the inter parietal sulcus. It forms an 4rch over the end of the superior temporal sulcus (Figs. 20 and 28). The angular arch is in direct continuity with the superior and middle temporal gyri and the posterior segment of it is continuous with the post-parietal gyrus when that gyrus is present.
The gyrus is present only when the middle temporal sulcus bends upward and terminates in the parietal lobe; in which case this gyrus curves over and closes that sulcus (not figured). It connects the posterior ends of the inferior and middle temporal gyri and also blends with the superior occipital gyrus. The angular, post-parietal and superior occipital gyri on the left side, according to Mills and others, constitute the center for visual memories. The studies of A. W. Campbell render it probable that the visual cortex does not extend into the parietal lobe of man at all. The receptive visual center for macular vision (vision in the macula lutea of the retina) is like wise located in the angular or post-parietal gyrus by Mills, but it is probably situated on the medial surface of the occip ital lobe; unlike the memory center, this is present in both hemispheres.
The whole parietal lobe, excepting the frontal half of the posterior central gyrus, lies in the psychic sensory region.
3. Occipital Lobe (Lobus occipitalis, Figs. 20, 26, 28, 31 and 34).—The occipital lobe forms the posterior pole of the hemi sphere. With the parietal and temporal lobes it is directly con tinuous, being marked off from them by an imaginary line drawn from the preoccipital notch to the occipito-parietal sulcus. This sulcus, on the convex surface, bounds it to the extent of about an inch; rarely, the anterior and transverse occipital sulci bound it in front. On the medial and basal surfaces of the hemi sphere the occipital lobe extends from the occipito-parietal sul cus and anterior calcarine fissure. to the preoccipital notch, and is separated from the temporal lobe on the basal surface by an imaginary line drawn from this notch toward the posterior end of the corpus callosum, to the isthmus of the limbic lobe. The occipital lobe has the form of a triangular pyramid whose borders are the supero-medial, the infero-lateral and the medial occipital borders of the cerebral hemisphere. Those borders meet at its apex, the occipital pole.
The followers of Eberstaller almost limit the occipital lobe to the tentorial and medial surfaces of the hemisphere; they describe the collateral fissure as the infero-lateral boundary on the tentorial surface. This extends the temporal lobe to the
occipital pole.
The occipital lobe is somewhat retrogressive in man, though present only in apes and men (Cunningham). It makes its appearance at the fourth month in utero, and is distinctly out lined by fissures on all three surfaces at the sixth month, when it resembles the same lobe in the adult ape's brain (Cunningham Memoirs). After the sixth month the fissural boundaries largely disappear from the human brain on the convex and basal surfaces; hence, the artificial boundaries in the adult. Its retrogressive character probably accounts for the great varia bility in the sulci and gyri of the occipital lobe. On the convex surface they are usually as follows: {Ramus occipitalis of interparietal sulcus Sulci Transverse occipital (s. occipitalis transversus) Lateral occipital (ss. occipitales laterales).
G Yr 1 i f Superior occipital (gg. occipitales superiores) Lateral occipital (gg. occipitales laterales).
The lateral occipital sulcus is the only one properly belonging to the convex surface of the occipital lobe (sometimes there are two of them). It divides that surface almost equally into a superior and lateral gyrus, both of which may be double. The sulcus begins near the supero-medial border. It follows a meridian which is nearly parallel with the infero-lateral border of the lobe and runs forward, often presenting one interruption, to the occipito-temporal boundary line. Posteriorly, it may bifurcate and form the short lunate sulcus (s. simians), which represents the affenspalte of the ape (Elliot Smith). When the lateral occipital sulcus is double there are two lateral gyri (Figs. 20, 27 and 28).
The occipital limb of the interparietal sulcus descends in the occipital lobe a variable distance (Fig. 2o). It is not always continuous with the horizontal limb. Running about an inch from the supero-medial border of the hemisphere, it passes the occipito-parietal sulcus, from which, it is separated by the arcus occipito-parietalis, and bifurcates in the superior occipital gyrus into two more or less oblique branches, constituting the transverse occipital sulcus. The medial end of the transverse sulcus may or may not cut the supero-medial border of the lobe; the lateral branch sometimes descends along the occipito temporal boundary. If the transverse and lateral sulci fail to reach the longitudinal fissure, a perpendicular gyrus forms the supero-medial border of the occipital lobe called the gyrus descendens.