Twelve pictures were obtained describing dissection steps of the lateral aspect of a left brain hemisphere.
After brain preparation, some classical landmarks were recognized. The deepest and longest lateral sulcus coursed slightly ascending and posteriorly in between the frontal, parietal, and temporal lobes. The central sulcus, separating the post- and pre-central gyrus with its omega shape related to the primary motor area of the hand, was identified (Fig. 1).
After peeling off the cortex, white fibers making a U-turn at the bottom of the sulci were visualized; these “U fibers” connect adjacent areas for fast connection or recovering in case of deep fasciculus interruption. The “U” shape of the pars opercularis and the “V” shape of the pars triangularis at the inferior frontal gyrus were recognized. Fibers curving around the insula connecting the inferior frontal to the superior and middle temporal gyri were visualized, and these corresponded to the long segment of the superior longitudinal fasciculus III (SLFIII or Arcuate fasciculus). Connecting vertically the superior frontal gyrus to the pars opercularis and triangularis, the frontal aslant tract (FAT) was exposed. The insular cortex was cautiously removed showing the 5 insular gyri, its central sulcus, and its inferior tip: the limen insulae (Figs. 2 and 3).
Under the insular cortex, the claustrum was identified as was the external capsule just underneath. From the pars opercularis to the temporo-occipital notch, the long segment of SLFIII was completely freed; its anterior terminations merging with the inferior ones of the FAT (Fig. 4).
Peeling of the external capsule led to stripes that exposed the lenticular body and particularly its lateral part – the putamen –. At the limen insulae, just below the ventral claustrum, fibers coursing between the frontal pole and the orbitofrontal cortex met in a narrow “temporal stem” and belonged to the uncincate fasciculus (UF). At the same level, fibers connecting the frontal and the occipital poles belonged to the inferior occipitofrontal fasciculus (IFOF). Just below the posterior segment of the IFOF segment were horizontal fibers from the temporal to the occipital poles that corresponded to the inferior longitudinal fasciculus (ILF; Fig. 5).
All of these fasciculi were preserved and the peeling continued at the central core region to enter the lenticular body distinguishing the putamen and the pallidum, the latter was a little bit tougher in consistency than the putamen. Medial to the lenticular body, vertical fibers accompanied by tiny perforating vessels were seen, and these belonged to the internal capsule. A dense block of white matter was visualized medial to the SLFIII and corresponding to a crossing fiber area formed by the internal capsule, the SLFIII, and the corpus callosum fibers. Below the claustrum, at the limen insulae, the close superimposition of IFOF and UF fibers was visualized (Figs. 6 and 7).
The removal of all SLFIII curving fibers exposed centrifugal fibers coming from the central core that corresponded to the corona radiata. The latter is also formed by the merging of both the external and internal capsule fibers at the top of the lenticular body. Below the lenticular body, transversal fibers connecting the two hemispheres were visualized, and corresponded to the anterior commissure. At the inferior posterior limit of the lenticular body, fibers turning anteriorly to make a loop corresponded to the optic radiations. The vertical segment of the SLFIII, the posterior segment of the IFOF, the anterior commissure, and optic radiation fibers were the components of the multi-layer white matter complex called stratum sagittale. (Figs. 8 and 9).
Having released all association fibers, fibers spreading from the internal capsule to the cortex were visualized, and corresponded to the whole corona radiata carrying sensorimotor information from or towards the limbs. At the level of the corona radiata, a large horse shoe shape bulge corresponded to the lateral ventricle. Just underneath the internal capsule, the grey of the thalamus was seen. Inferiorly, the median handlebar of the anterior commissure and the optic tracts were delimited as medial lateral white fibers in the middle of small grey triangular spaces: the first anterior one – substantia inominata – connected to the thalamus and the accubens nucleus; the second posterior one connected to the amygdala body. The optic tracts emerged from the lateral geniculate body at the inferior aspect of the thalamus and were followed by the optic radiations making an anterior loop described by Meyer [7]. Another grey structure was seen superior to the thalamus and inferior to the assumed position of the lateral ventricle: the caudate nucleus (Figs. 10 and 11).
A last peeling of white fibers all around the diencephalic deep grey nuclei led to enter the lateral ventricle with choroid plexuses within. A thin layer of fibers from the internal capsule was kept to expose the corticospinal tract (CST). Stripes were made through the optic radiations and a thin layer of tapetal fibers to localize the atrium and the temporal horn of the lateral ventricle. Corpus callosum fibers were cut above the roof of the fontal horn and the body of the lateral ventricle. ILF fibers coursing along with optic radiations were visible (Fig. 12).