The anterior border is bent downward and then backward sweeping through 18o degrees of flexion (Fig. 35). It tapers down to a sharp edge called the rostrum. A very thin sheet-like extension of the rostrum, called the lamina rostralis, proceeds backward from the beak and becomes continuous with the lamina terminalis. The transverse fibers of the rostrum in the hemisphere form the floor of the anterior horn of the lateral ventricle. Running downward on either side of the rostrum is a low ridge, continuous with the stria longitudinalis medialis, which constitutes the gyrus subcallosus. Each gyrus sub callosus, after passing across the anterior perforated substance, ends in the uncus of the hippocampal gyrus.
Genu and Truncus (Fig. 35) .—The down-turned anterior part of the corpus callosum is the genu. It joins the rostrum to the main body, the truncus. The genu forms part of the anterior boundary of the cerebral cavity; the truncus forms the roof. Fibers uniting the frontal lobes of the cerebrum pass through the genu, and in the hemisphere, bound the anterior horn of the lateral ventricle above and in front. Those fibers arching for ward and forming the roof of the anterior horn are called the forceps minor. The forceps major, composed of fibers from the splenium which bend backward into the occipital lobe, lies in the roof and inner wall of the posterior horn and produces the eminence called the bulb (Fig. 49).
Each lateral extremity of the corpus callosum is overhung by the gyrus cinguli, which covers the lateral longitudinal stria. Inclosed between the gyrus cinguli and corpus callosum is the callosal fissure (ventricle of the callosum) . The lateral extremity of the corpus callosum, within the cerebral hemisphere, inter mingles with the superior lamina of the internal capsule and thus stretches entirely across the fore-brain cavities (Figs. 37 and 54).
The boundaries of the general cavity of the fore-brain may be given as follows: Roof (base of wedge)— Corpus callosum.
Floor (edge of wedge)— Tegmenta of mid-brain, Posterior perforated substance of mid-brain, Tuber cinereum, Infundibulum, Optic chiasma.
Lateral wall (beveled surf ace)— Internal capsule (superior lamina), Caudate nucleus, Stria terminalis, Thalamus.
Anterior wall (border of wedge)— Lamina terminalis, Anterior commissure, Genu of corpus callosum.
Posterior wall— Posterior commissure with cerebral aqueduct beneath it, Pineal body, Corpora quadrigemina of mid-brain, Transverse fissure of cerebrum, containing the chorioid tela of third ventricle, Splenium of corpus callosum.
The fore-brain cavity thus bounded is subdivided by two partitions (Figs. 35, 46 and 54). The body of the fornix, together with the chorioid tela of the third ventricle and the roof epithelium of the third ventricle, forms a horizontal partition which divides the cavity into an upper and lower chamber. The
superior chamber is divided into two lateral chambers, the lateral ventricles, by a double vertical partition, the septum pellucidum. The inferior chamber is the third ventricle.
The body of the fornix (corpus fornicis, Figs. 35 and 47) is a triangular sheet of fibers, whose base is attached to the under surface of the splenium of the corpus callosum, and whose bifid apex extends forward to the rostrum and the anterior com missure. Its lateral borders rest on the thalami, the chorioid tela alone intervening (Fig. 54). And the narrow chamber between the thalami, the third ventricle, is separated from the broader, superior part of the fore-brain cavity by the body of the fornix together with the chorioid tela and layer of epithelium. The body of the fornix is produced by the approximation of two bundles of white fibers, one belonging to each hemisphere. These bundles are the crura of the fornix.
The crus fornicis (Figs. 35, 43 and 49) may be traced from the uncus and the hippocampus, its chief origin, upward through the inferior horn and into the floor of the body of the lateral ventricle, where it unites with its fellow of the opposite side in forming the body of the fornix. At the apex of the body of the fornix, which is the anterior end, the bundles again separate and become the columnx of the fornix. The crura are united at the back part of the body of the fornix by a few transverse and oblique fibers which form the lyre, or commissura hippocampi (Fig. 47). The commissure is best seen when the corpus callosum and fornix are viewed from below; its fibers connect each crus of the fornix with the hippocampus and uncus of the opposite side.
The colurrmx fornicis (Figs. 35, so and 51), one on either side pass down in front of the thalami, bounding the foramina inter ventricularia (Monroi); and then descend to the corpora mammillaria, at the base of the brain. On the way down the free part of each columna (pars libera) passes behind the anterior commissure, beyond which (as pars tecta) it pierces the inner part of the thalamus of the same side. The fibers of the columna fornicis for the most part terminate in the medial nucleus of the corpus mammillare, from which other fibers take their origin, forming the fasciculus mammillaris princeps. This bundle divides Y-like; the anterior branch is the fasciculus mammillo-thalamicus (Vicq d'Azyri) and ascends to the anterior nucleus of the thalamus; the posterior bundle is the fasciculus mammillo-tegmentalis and probably ends in the stratum griseum centrale and nucleus tegmenti profundus of the mid-brain.