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The Base of the

orbital, sulcus, frontal, surface, inferior, lobe and gyrus

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THE BASE OF THE The basal or inferior surface of the fore-brain comprises the inferior surface, first, of the end-brain, including the pars optica hypothalami and the cerebral hemispheres; and, second, of the inter-brain, which embraces the pars mammillaris hypothalami. It is completely exposed only when a section is made through the mid-brain and the rhombencephalon removed (Figs. 21 and 31). This should now be done with a thin, moistened brain-knife. Make the section from before backward and upward, at a right angle to the axis of the mid-brain. Now notice, first, the section of the mid-brain and, just anterior to that, the median struc tures of the fore-brain, occupying the center of the field; and, second, the surrounding inferior surface of the cerebral hemi spheres. The latter form the very large peripheral zone.

The base of the cerebral hemisphere extends from the frontal to the occipital pole. In front it is composed of the orbital area bounded by the medial orbital and superciliary borders; and, posteriorly, is made up of the tentorial area, which is bounded, laterally, by the infero-lateral border, and, medially, by the chorioidal fissure and the medial occipital margin of the hemisphere (Fig. 31). The orbital area embraces the inferior surface of the frontal lobe and of the island, and the whole ol factory lobe; while the inferior surface of the temporal and occipital' lobes, and the gyrus hippocampi and fascia dentata of the limbic lobe are included in the tentorial area.

Frontal Lobe, Inferior Surface (Fig. 31) .—The inferior surface of the frontal lobe, resting on the orbital plate of the frontal bone, is often called the orbital lobe. It is separated from its fellow by the longitudinal fissure of the cerebrum, and is bounded behind by the lateral fossa and lateral fissure of the cerebrum, overlapped by the temporal lobe. More accurately, the pos terior boundary is the anterior perforated substance and the anterior part of the circular sulcus. The orbital lobe is con cave transversely and is divided by the triradiate or H-shaped sulcus orbitalis, made up of the medial orbital, the transverse orbital and the lateral orbital sulci; and by the olfactory sulcus, which is close to the longitudinal fissure and nearly parallel with it. Five gyri are thus formed:

I Straight (g. rectus) Medial orbital (g. orbitalis medialis) • Gyri Lateral orbital (g. orbitalis lateralis), not constant Anterior orbital (g. orbitalis anterior) Posterior orbital (g. orbitalis posterior).

The gyrus rectus (Fig. 3T) forms the medial border of this surface. It is separated from the medial orbital gyrus by the sulcus olfactorius in which lie the olfactory bulb and tract. Over on the medial surface it forms part of a marginal gyrus and it joins the superior frontal at the frontal pole. Pos teriorly the gyrus rectus is separated from the parolfactory area (of Broca) by a slight furrow, the anterior parolfactory sulcus.

The medial orbital gyms lies between the sulcus of the same name and the sulcus olfactorius (Fig. 31). It extends from the frontal pole to the anterior perforated substance and the island. The anterior and posterior orbital gyri lie within the H-shaped orbital sulcus separated from each other by the transverse orbital sulcus. The former is continuous with the frontal gyri at the superciliary border; the latter is only partially separated, behind, from the island anterior circular sulcus; the posterior orbital gyrus is likewise continuous with the pos terior end of the lateral orbital gyrus and with the orbital portion of the inferior frontal. The lateral orbital gyms, which is a distinct gyrus only when the lateral orbital sulcus is long, is situated lateral to the H-shaped sulcus. It is continuous with both middle and inferior frontal gyri at the superciliary border of the hemisphere.

The Island (of Reil), Inferior Surface (Fig. 3i).—If the an terior part of the temporal lobe be removed, the under surface of the island (insula) is brought into view. The circular sulcus bounds it on two sides and separates it from the posterior orbital gyrus, in front; and from the temporal lobe, behind. Laterally it is separated from the frontal and the parietal parts of the operculum by an antero-posterior cleft continuous with the lateral cerebral fissure.

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