LOBES AND GYRI OF THE MEDIAL AND BASAL SURFACES.
All four cerebral lobes, with which we have now become somewhat intimately acquainted on the dorso-lateral aspect of the hemisphere, are continued onto the medial and partly also onto the basal surface. They do not extend, however, over the entire medial surface, but bound a large annular tract that belongs to the rhinencephalon. Let us first examine the defining fissures and suici.
The sulcus cinguli, or calloso-marginal fissure, begins beneath the rostrum of the corpus callosum. It runs forward, around the knee, and then backward, more or less parallel with the corpus callosum, as far as the splenium. Here it bends at a blunt angle upward towards the superior margin of the hemisphere as the ramus marginalis. During its entire course, several and sometimes deep incisions branch off, upward as well as downward. In front of the obtuse bend, approximately over the middle of the corpus callosum, the fissure usually sends a side branch, the sulcus fiaracentralis, upward. Another branch, the sulcus sufiraorbilalis (Broca), is given off at the level of the genu. Finally, a third fissure, the sulcus subfrarietalis, which represents a continuation of the chief furrow, runs backward and around the splenium corporis callosi. Immediately beneath the knee and the rostrum of the callosum, the sulcus corporis callosi begins, at first as only a shallow fissure. It often appears there as the prolongation of the sulcus parolfactorius posterior (see Rhinencephalon, page 26), then continues around the genu, closely follows the convex surface of the corpus calloslim, runs around the splenium and continues into the fissura hippocampi, the deep cleft that runs from behind and above forward and downward.
In the posterior part of the medial surface of the hemisphere, beginning about mid way between the turned-over end of the central fissure and the occipital pole, the deep fissura runs obliquely forward and downward, behind the lower end of the subparietal branch of the sulcus cinguli, as fal• as the region beneath the splenium corporis callosi. In the lower part, at about the level of the splenium, the furrow is joined at an acute angle by the deep fissura calcarina. The latter, slightly arched and somewhat above the medial border, extends backward toward the occipital pole, where it may end as a simple groove, or, as is usually the case, in two widely divergent branches. Occasionally the calcarine fissure overruns the occipital pole and terminates on the dorso-lateral surface of the heniisphere. The stem formed by the union of the parieto-occipital and calcarine fissures extends downward and close behind the hippo campal fissure, without, however, joining the The fissura collaleralis begins at the level of the occipital pole, below the calcarine fissure, and passes forward below the common stem of the parieto-occipital and calcarine fissures. Its continuation into the anterior part of the temporal lobe constitutes the fissura rhinica, whose front end is known as the incisura temporalis (Schwalbe). Below the collateral fissure is the sulcus temporalis inferior.
By means of the foregoing fissures, the following parts are defined. The tract occupying the front part of the medial surface outside the sulcus cinguli belongs to the frontal lobe, more particularly to the superior frontal convolution. It extends back ward beyond the paracentral sulcus, its posterior limit being a line drawn from the medial end of the central sulcus, between the paracentral and marginal rami of the sulcus cinguli. to the last-named fissure. The tract between the paracentral and mar
ginal branches of the sulcus cinguli is called the lobulus paracentralis. Here is found the transition of the gyrus centralis anterior into the gyrus centralis posterior. The larger part of the paracentral lobule belongs to the precentral convolution. Behind the tract belonging to the frontal lobe, a region broadens out which belongs to the parietal lobe. It lies above the sulcus cinguli and its prolongation, the subparietal fissure, and is bounded behind by the parieto-occipital fissure. The entire tract, between the marginal arm of the sulcus cinguli in front, the subparietal fissure below and the parieto-occipital fissure behind, constitutes the firaecuneus or quadrate lobule. Between the parieto-occip ital and calcarine fissures lies the cuneus, which belongs to the occipital lobe. Below the calcarine fissure, between it and the collateral fissure, lies another part of the occip ital lobe known as the gyrus lingualis. On the basal aspect of the hemisphere, below the collateral fissure, the gyrus fusiformis extends as a part of the temporal lobe. It is also called the gyrus A large annular tract belonging to the rhinencephalon is enclosed by the foregoing convolutions and lobes. Externally it is bounded by the sulcus cinguli, the common stem of the parieto-occipital and calcarine fissures, the front end of the collateral fissure and the fissura rhinica. The inner boundary is contributed by the sulcus corporis callosi and the fissura hippocampi. In its entirety, this tract constitutes the gyms fornicatus or the limbic lobe. It is subdivided into the gyrus cinguli, which arches around the corpus callosum, and the gyrus hippocampi, which is included between the hippocampal fissure on the one side and the collateral and rhinal fissures on the other and hooks around the anterior end of the hippocampal fissure to form the uncus. The gyrus cinguli and the gyrus hippocampi are continuous, behind and below the splenium, by means of the isthmus gyri fornicati.
Turning once more to the basal surface, in the posterior and larger division, we note the sulci and gyri already mentioned:—the fissura hippocampi, the fissura parieto occipitalis and calcarina, joining to form a common stem, the fissura collateralis, the fissura rhinica, the sulcus temporalis inferior and the convolutions extending between these furrows. The anterior and smaller division of the basal aspect belongs to the lobus frontalis, being known as its orbital surface. Near the medial border of the latter, the straight sulcus olfactorius runs forwards and somewhat medially and lodges the olfactory bulb and tract. The sulcus,is deep and almost always extends farther forwards than the anterior end of the bulbus olfactorius. Behind, it divides into a: ramus medialis and lateralis, which embrace the tuberculum olfactorium. Lateral to the olfactory fissure lie some furrows of uncertain number and arrangement, the sulci orbitales. By their union the most varying patterns are produced, including H - , X - , L - , T - , K- and Z like forms. Medial from the sulcus olfactorius extends the gyrus rectus. The gyri orbitales are bounded by the orbital fissures.