Ectorhinal Sulcus (Incisura temporalis, Figs. 31 and 35).— Midway between the temporal pole and the hook-point of the hippocampal gyrus is a slight notch, called the ectorhinal sulcus, which represents an important lateral boundary of the rhinen cephalon in animals with highly developed sense of smell. It indicates in man the boundary between the hippocampal and fusiform gyri. A half inch behind the ectorhinal sulcus is the anterior end of the collateral fissure.
Fissura Collateralis (Figs. 31 and 34).—The collateral fissure extends in a somewhat curved course from near the temporal pole almost to the occipital pole. Its anterior two-thirds separates the hippocampal from the fusiform gyrus; its pos terior one-third completes the medial and upper boundary of the fusiform gyrus and separates it from the gyrus lingualis.
Inferior Temporal Sulcus (Fig. A.—Only one sulcus belongs wholly within the inferior surface of the temporal and occipital lobes. It extends from a point near the occipital pole forward along the infero-lateral border of the hemisphere almost to the temporal pole, and incompletely separates the inferior temporal gyrus and the lateral occipital gyrus from the gyrus fusiformis. Very frequently the sulcus has two or more interruptions. It may be called the occipito-temporal sulcus.
Gyrus Fusiformis.—One gyrus only is found entirely within the inferior temporo-occipital region (Figs. 31 and 35). That is the fusiform (occipito-temporal gyrus). It extends from near the occipital pole forward and forms the temporal pole. The posterior nine-tenths of its medial boundary is formed by the collateral fissure and the anterior one-tenth by an imaginary line and the ectorhinal sulcus; laterally, it is bounded by the inferior temporal sulcus.
Gyrus Lingualis.—The gyrus lingualis lies above and medial to the posterior one-third of the collateral fissure; inferior and lateral from the calcarine fissure. It is continuous with the gyrus hippocampi of the limbic lobe in front. The gyrus lingualis (Fig. 29) forms nearly all of the medial occipital border of the hemisphere. It contains a part of the receptive visual center (Figs. 75 and 77).
Limbic Lobe (Lobus Limbus), Inferior Part—The gyrus hippocampi of this lobe is visible on the inferior surface of the fore-brain (Fig. 31). Notice how this crescentic gyrus embraces in its concavity the section of the mid-brain. It is separated
from the fusiform gyrus by the collateral fissure and the ectorhinal sulcus; and bounded medially by the hippocampal fissure. The anterior end of the gyrus is flexed inward and backward over the end of the hippocampal fissure and the whole anterior part constitutes the uncus hippocampi. The region of the uncus is somewhat irregular and, in a four-month embryo, presents the gyrus circumambiens and the gyrus semilunaris described by Retzius. It represents the greater part of the lobus pyraformis of osmatic mammals and is probably the chief receptive center of smell; it receives the lateral stria of the olfactory tract and fuses with a low oblique ridge, the gyrus diagonalis of Broca, which in front is continuous with the gyrus subcallosus (or peduncle of the corpus callosum).
If the hippocampal gyrus be drawn downward somewhat, a rudimentary gyrus may be seen in the floor of the hippocampal fissure, between the gyrus hippocampi and the fimbria. That gyrus is the fascia dentata. The fascia dentata is continuous posteriorly with two small gyri, the fasciola cinerea and gyrus subsplenialis, both under the splenium of the corpus callosum, by means of which it is linked to the gyrus supracallosus; ante riorly the fascia dentata sinks into the concavity of the uncus and bends at a right angle, the angulus fascice dentate, then it winds medially and upward over the free end of the uncus, as the pars transversa of the dentate fascia (band of Giacomini) ; it fades away on the superior surface of the uncus. The pars transversa of the fascia dentata marks the boundary line between the gyrus intralimbicus behind it and the nucleus amygdalce, which is in front of it.
Having studied the basal structures of the cerebral hemi spheres, it is now in order to examine the median structures in the inferior surface of the fore-brain. They occupy the inter peduncular or hypophyseal region. They constitute the hypothalamus and form part of the floor of the third ventricle.
The hypothalamus is the name applied to the cerebral struc tures under the thalamus. Posteriorly it blends with the mid brain. Its free portion is divided into two parts, viz., the pars optica hypotbnlami and the pars mammillaris hypothalami. The former belongs to the telencephalon, the latter to the diencephalon. They include the following: Lamina cinerea terminalis.