2. The parietal lobe (lobus parietalis) is situated behind the central sulcus and above the posterior limb of the lateral fissure of the cerebrum (Figs. 26 and 34). From the curve near the posterior end of the latter to the occipito-parietal sulcus the lobe is separated from the temporal, below, and the occipital, behind, by an imaginary line. This imaginary line runs back ward parallel with the infero-lateral border of the hemisphere to the boundary of the occipital lobe; and then, obliquely up ward toward the supero-medial border in a line drawn from the preoccipital notch to the occipito-parietal sulcus. Extending over the supero-medial border, the lobe on the medial surface is inclosed between the occipito-parietal sulcus behind and the marginal part of the sulcus cinguli in front, and is bounded antero-inferiorly by the subparietal sulcus.
On the convex surface of the hemisphere the parietal lobe possesses the following sulci and gyri (Figs. 27 and 28).
(s. interparietalis) four parts— Inferior post-central (s. post-centralis inferior) Superior post-central (s. post-centralis superior) Horizontal limb (ramus horizontalis) Occipital limb (ramus o'ccipitalis).
Sulci I Upturned ends of fissure (f. cerebri lateralis) Superior temporal sulcus (s. temporalis superior) Middle temporal sulcus (s. temporalis medius). Posterior central (g. centralis posterior) Superior parietal lobule (1. parietalis superior) Gyri Inferior parietal lobule (I. parietalis inferior) Supramarginal (g. supramarginalis) Angular (g. angularis) Post-parietal (g. post-parietalis).
The interparietal sulcus (Figs. 20 and 27) is the only one belonging to the parietal lobe. The inferior and superior post central sulci, constituting its anterior parts, are parallel with the central sulcus and are located a half or three-quarters of an inch behind it, separated from the central sulcus by the gyrus centralis posterior. The post-central sulci are often not con tinuous. The inferior is about twice the length of the superior, in this resembling the central sulcus, and usually it is joined at its upper end to the horizontal limb of the interparietal sulcus. The horizontal part of the sulcus lies about an inch below the supero-medial border of the hemisphere with which it is parallel; it separates the superior parietal lobule from the in ferior parietal lobule and is continued as ramus occipitalis into the occipital lobe where it bifurcates. The horizontal part of this sulcus has one superior and two inferior rami; the trans verse parietal ramus runs toward the supero-medial border, bisecting the superior parietal lobule; the intermediate rami descend, the first is opposite the transverse parietal and be tween the up-turned ends of the lateral fissure and first tem poral sulcus, the second intermediate ramus descends behind the first temporal sulcus.
The posterior central gyrus reaches from the posterior limb of the lateral fissure upward and backward, between the central and post-central sulci, to the longitudinal fissure of the cere brum (Fig. 28). It is joined to the anterior-central gyrus around the ends of the central sulcus by superficial annectant gyri (gyri transitive) and sometimes is connected with it by a buried gyrus (g. profundus transitivus) which, deeply, separates the superior from the inferior part of the central sulcus. The annectant gyrus which closes the central sulcus superiorly and links together the central gyri is the paracentral lobule (lobulus paracentralis); the fronto-parietal part of the operculum joins them below the central sulcus. The posterior central gyrus and paracentral lobule constitute the receptive area of common sensation, the somcesthetic area, so far as it extends on the convex surf ace.
The superior parietal lobule (Figs. 20 and 28) forms the supero-medial border of the hemisphere from the superior central to the occipito-parietal sulcus. It is separated from the inferior parietal lobule by the horizontal part of the interparietal sulcus; posteriorly, it is joined to the occipital lobe by a curved annectant gyrus, called the arcus occipito-parietalis, which closes the superior end of the occipito-parietal sulcus; and, over the supero-medial border, it is continuous with the prwcuneus of the medial surface. In the prxcuneus and the superior parietal lobule Mills locates, the stereognostic center (Figs. 76 and 77). The Inferior Parietal Lobule.—The inferior parietal lobule is incompletely divided into two or three gyri. Named from before backward they are as follows: The supramarginal, the angular and the post-parietal (Figs. 20 and 28).
The supramarginal gyrus arches over and closes the upturned end of the posterior ramus of the lateral fissure of the cerebrum (Fig. 28). The anterior segment of the arch is continuous with the posterior central gyrus, the posterior segment of the arch fuses with the angular gyrus, behind, and the superior temporal gyrus, below. The supramarginal gyrus is partially separated from the angular gyrus by the sulcus intermedius primus. This gyrus belongs in the psychic sensory area, probably containing the center of the muscle sense (Fig. 76).