The Olfactory Triangle and the Parolfactory Area (of Broca).— The triangular portion of the cortex between the medial and lateral olfactory strie, called the triangle (trigonum olfactorium) is continuous medially with the area parolfactoria. The medial stria marks the boundary between them (Figs. 31 and 33). Both are bounded behind by the sulcus parolfactorius posterior (transverse part), and the oblique part of the same sulcus separates the parolfactory area from the gyrus subcallosus (peduncle of the corpus callosum). The olfactory triangle is a wedge-shaped mass of gray substance (olfactory tubercle) at the caudal end of the olfactory tract. It is prolonged for ward in the olfactory sulcus, as the dorsal border of the olfactory tract; that prolongation constitutes the cortex of the tract. The olfactory triangle is retrogressive in the adult brain. At the fifth month in utero it is more prominent; it is divided into two definite parts, viz., the gyrus olfactorius medialis, persisting as parolfactory area, and the gyrus olfactorius lateralis, which trends lateralward to the threshold of the island; there, it forms a sharp angle and, proceeding medianward, ends at the uncus hippocampi. A small branch of the lateral olfactory gyrus disappears on the orbital surface of the frontal lobe. The medial and lateral olfactory gyri are faintly indi cated on the adult brain by the medial and lateral stria. The area parolfactoria (Broce) is limited in front by a slight curved depression, the sulcus parolfactorius anterior. On the medial surface it ascends between the anterior and posterior parolfactory sulci to the corpus callosum and becomes con tinuous with the lateral part of the gyrus supracallosus.
The anterior perforated substance (substantia perforata anterior) of the pars posterior rhinencephali requires further mention (Fig. 31). It is separated from the triangle by a very faint groove, the posterior sulcus parolfactorius. Medially, it is in direct continuity with the tuber cinereum. The optic tract bounds it, postero-medially. Laterally, it forms the limen insulce in the floor of the fossa cerebri lateralis, where it is overlapped by the temporal lobe. Superiorly, it is continuous with the base of the corpus striatum. Coursing along the inner and outer borders of the anterior perforated substance are, respectively, the gyrus diagonalis and lateral olfactory stria, which converge and meet in the hippocampal gyrus. The perforations of this area are for the antero-lateral ganglionic arteries.
The olfactory triangle, the cortex of the tract, the parol factory area and the anterior perforated substance, together with the septum pellucidum, constitute a complete relay in the olfactory path; they contain the bodies of the third order neurones.
Tentorial Area of the Basal Surface (Figs. 31, 28 and From the temporal pole backward, the basal surface of the cerebral hemisphere presents three nearly parallel gyri, viz.,
the first includes the inferior temporal and lateral occipital gyri, which form the infero-lateral border; the fusiform gyrus is the middle one; and, third, the gyrus lingualis and the gyrus hippocampi, which lies next the mid-brain. The last belongs to the gyrus fornicatus of the limbic lobe; it is continuous, posteriorly, with the lingual gyrus, which forms a part of the medial occipital border of the cerebral heimsphere. The fusi form and inferior temporal gyri belong to the inferior surface of the temporal and occipital lobes. These two lobes are directly continuous with each other on their inferior surfaces, and are only separated arbitrarily by an imaginary line drawn from the preoccipital notch to the anterior end of the calcarine fissure. They are only partially separated from the gyrus hippocampi; the ectorhinal sulcus (s. rhinalis) and the anterior part of the collateral fissure lie between the temporal lobe and the hippocampal gyrus of the limbic lobe; while the inferior surface of the occipital lobe is continuous with the gyrus hip pocampi but is separated from the gyrus cinguli, of the limbic lobe, by the anterior calcarine fissure. The fissures and sulci of the tentorial area are the following: Chorioidal fissure (f. chorioidea) Hippocampal fissure (f. hippocampi) Ectorhinal sulcus (s. ectorhinalis) Collateral fissure (f. collateralis) Inferior temporal sulcus (s. temporalis inferior) Calcarine fissure (f. calcarina) end of it.
The chorioidal fissure (f. chorioidea) forms a part of the medial boundary of the tentorial area (Figs. 31 and 34). At the surface it appears to be identical with the hippocampal fissure; but, upon looking deeper, the two are found to be separated by the fascia dentata and the crus of the fornix. This fissure is separated from the inferior horn of the lateral ventricle, only by a layer of epithelium, derived from the roof plate of the telencephalon. It contains the chorioid plexus of the inferior horn.
Hippocampal Fissure (F. hippocampi, Fig. 34).--Between the mid-brain and concave border of the hippocampal gyrus is the crescentic fissure known as the hippocampal fissure. The fissure in front is closed by the uncus. It extends backward to the splenium of the corpus callosum where, in the adult, it is continuous with the furrow behind and above the corpus callosum, called the callosal sulcus. The hippocampal is a true fissure as it is a cleft between the mesencephalon and the telencephalon. In its floor lie the fascia dentata and the fimbria hippocampi (crus fornicis); anterior to these structures it is continuous with the chorioidal fissure. Though the hippo campal fissure is parallel with it, it does not produce the hippo campus seen in the inferior horn of the lateral ventricle (see medial surface of the cerebral hemisphere).