INTERNAL CONFIGURATION OF THE TELENCEPHALON.
The examination of the inner configuration of the end-brain is carried out most advantageously in the following manner. A brain is laid on its base and the removal of the hemispheres is begun. This is ac complished by passing horizontally, with slow continuous stroke, a long brain knife from the convex lateral surface of the hemisphere as far as the longitudinal cerebral fissure. In this manner first the right and then the left hemisphere are removed, from above downward, in disk-like pieces about one centimeter thick. The last horizontal section should fall about 5 mm. above the dorsal surface of the corpus callosum.
Each section distinctly exhibits two different substances—the white substance, light in color and situated in the interior, and the gray substance, which every where encloses the former and continues as a band at the periphery (Fig. 38). In the first pieces, the white substance is less voluminous than the gray. The deeper one cuts, however, the greater the amount of white substance revealed, and in the last section, passing imme diately above the callosum (Fig. 39), in each hemisphere is seen a large white medul lary field, the centrum semiovale (Vieussens), which peripherally is bounded by the gray band representing the cerebral cortex, the substantia cornea/is.
The substantia corticalis is not everywhere equally developed as to thickness, in this respect varying according to the region of the brain. In general', the cerebral cortex is more developed on the summit of the convolutions and less so at the bottom of the fissures, being thicker on the outer convex surfaces than on the medial and basal aspects of the hemispheres. The cortex reaches its maximum development in the upper part of the central convolutions and in the lobus paracentralis, and its minimum in the occipital pole. When closely examined, even with the unaided eye, one recognizes that the cerebral cortex is not homogeneous, but is composed of alternating gray and white strata arranged parallel with the surface. The white bands are known as Baillarger's stripes. The cortex of the occipital lobe, particularly around the calcarine fissure, exhibits this stratification quite distinctly. Here three layers are found, an outer and an inner gray stratum and, between them, a thin light band, the stripe of Vicq a' Azyr (Fig. 4o), or, since Gennari first described it, the stripe of Gennari. The explanation of this lamellation will be given later by the microscopical examination of the cerebral cortex (page 114).
In consequence of the removal of the upper part of the hemispheres, as already suggested, the corpus callosum comes plainly in view. The dorsal surface of this bridge
lies before us, while on each side it is separated from the overlying medial surface of the hemisphere by the sulcus corporis callosi.
The corpus callosum, or commissura cerebri magna, forms a white medullary mass that connects the hemispheres. Strands of transversely coursing fibres, the striae transversae, are seen on the surface of the truncus or body of the corpus callosum.
They penetrate the wall of the hemispheres and form the radiatio corporis callosi (Fig.
39). The callosal radiation includes an anterior, a middle and a posterior part. The anterior portion, the pars fronlalis, belongs to the callosal .knee or genu and connects the anterior parts of the frontal lobes. In consequence of the frontal lobes projecting beyond the genu, the connecting fibres sweep in curves far forward toward the frontal poles, forming a sort of tongs, the forceps an terior. The middle portion, the pars parietalis, belongs to the body of the corpus callosum and unites the parietal and temporal lobes of the two sides. The posterior portion belongs to the hind segment of the callosal body and the splenium and, as the pars lemporalis and pars occipitalis, connects the tem poral and occipital lobes. The callosal fibres arch far backward toward the oc cipital poles and form the forceps poste rior. The induseum griseum covers the upper surface of the corpus callosum as a thin investment, that presents two medial linear thickenings, and, on each side, a lateral one. The medial longitu dinal stripes, the striae longitudinales me diates or striae of Lancisii, are separated by a longitudinal furrow, the raphe cor poris callosi. The lateral stripes, situated within the corresponding sulcus corporis callosi, are the striae longitudinales laterales or the teniae &clew.
Now follows the opening of the lateral ventricles. Such parts of the hemispheres which still overlie the corpus callosum are removed as far as the level of the dorsal sur face of the callosum. On separating these parts with the fingers, in properly hardened brains, it is possible to demonstrate the radiatio corporis callosi, especially the forceps anterior and posterior. A pointed knife is now passed through the roof of the lateral ventricle, at the side of the body of the callosum and from 1-2 cm. behind the genu. The incision is lengthened straight forward as far as the level of the genu of the cal losum and backward, in a slightly outwardly convex curve, to a point behind the splenium. By gradually widening the opening medially and laterally the ventricle is exposed.