FISSURES OF THE MEDIAL AND TENTORIAL SURFACE To expose the medial surface of the cerebral hemispheres, a median sagittal section must be made through the connecting links of the hemispheres and the inter-brain, dividing the fore brain into lateral halves. Separate the lips of the longitudinal fissure of the cerebrum; drop the moistened brain-knife down onto the corpus callosum; and make one quick sweep of the knife toward you. Of the surface now exposed the middle one third is produced by section.
It is convenient to study the tentorial area of the basal surface with the medial surface (Fig. 31). In this medial and tentorial surface there are eight important sulci and four fissures (Fig. 34).
Of cingulum (s. cinguli) Callosal (s. corporis callosi) Subparietal (s. subparietalis) Sulci Occipito-parietal (s. occipito-parietalis) Inferior temporal (s. temporalis inferior) Ectorhinal (s. ectorhinalis) Parolfactory (ss. parolfactorius ant. and post).
Calcarine (fissura calcarina) Fissures Hippocampal (f. hippocampi) Chorioidal (f. chorioidea) Collateral (f. collateralis).
Sulcus Cinguli (Calloso-marginal Sulcus).—Beginning under the middle cut surface and extending in a curve forward, up ward, and backward, until it half encircles the corpus callosum; and then turning upward to the supero-medial border and ending just behind the central sulcus is the sulcus cinguli (Figs. 33 and 34). It separates the gyrus cinguli and a marginal gyrus, including the straight and superior frontal, from one another by its anterior part; and by its marginal end separates the paracentral lobule from the pnecuneus. The sulcus cinguli is usually interrupted by one annectant gyrus and often by two. These indicate its development in three separate parts. Several branches radiate from the cingulate sulcus toward the supero-medial border of the hemisphere; the most constant is the sulcus paracentralis, which rises a short distance in front of the marginal part and forms the anterior boundary of the paracentral lobule.
At its beginning under the corpus callosum, the sulcus cinguli is almost continuous with a small curved sulcus, which runs nearly vertically downward, called the anterior parolfactory sulcus (Figs. 34 and 35). Behind that little sulcus there is a small curved gyrus, the parolfactory area (of Broca), which is continuous with the lateral stria of the gyrus supracallosus; the parolfactory area is bounded behind by another slight sulcus, called the posterior parolfactory sulcus. The latter
separates the area parolfactoria from the gyrus subcallosus.
Subparietal Sulcus.—About one inch above and behind the posterior end of the corpus callosum there is an irregular sulcus, called the subparietal, which separates the gyrus cinguli of the limbic lobe from the prancuneus of the parietal lobe (Fig. 34). Sometimes it is continuous with the cingulate sulcus, at the junction of the marginal part; in most brains it is an independent sulcus having the shape of a very broad X.
The callosal sulcus is the deep furrow between the corpus callosum and the gyrus cinguli. It follows the convexity of the corpus callosum and was formerly called the ventricle of it (Fig. 34). The callosal sulcus, behind the corpus callosum, is con tinuous with the hippocampal fissure.
The sulcus (Figs. 20, 33 and 34), the inter nal part, extends downward from the supero-medial border to the middle of the calcarine fissure. The two form a lambda shaped fissure .< (Fig. 34); the lambda being tilted toward the frontal pole has one anterior and two posterior rami. The anterior ramus and the lower of the posterior rami constitute the calcarine fissure; the posterior superior ramus is the occipito-parietal sulcus. This latter sulcus cuts the supero medial border at the junction of the posterior one-sixth with the anterior five-sixths of that border; it is situated about two inches above the occipital pole, and lies one-sixth of an inch anterior to the point in the skull called the lambda. It sepa rates the parietal lobe from the cuneus of the occipital lobe. The occipito-parietal sulcus is a deep one. In the embryo the primary occipito-parietal fissure produces an eminence in the posterior horn of the lateral ventricle (Cunningham). It is then a true fissure. But that primitive fissure and the ventricu lar eminence entirely disappear, and the adult surcus is a secondary and superficial furrow, hence it is properly called a sulcus and not a fissure. At the inferior end of the occipito parietal sulcus a buried annectant gyrus, the gyrus cunei, separates the occipito-parietal sulcus from the calcarine fissure, with which superficially it is continuous.