The frontal stalk of the thalamus originates in the lateral nucleus of the thalamus. It issues from the anterior part of the nucleus and, running through the frontal part of the cap sule, terminates in the caudate nucleus and in those parts of the frontal gyri in which the fronto-pontal tract takes its origin. This is an afferent tract, conducting impulses to the caudate nucleus and frontal cortex; but it is probably not a sensory tract.
Disregarding for the moment the direction of growth and conduction, we may say that the fibers of the internal capsule radiate toward the cortex as soon as the striate body is passed; they form the capsular radiation; and, together with the radia tion of the corpus callosum, they form the corona radiata, which is shown in frontal sections of the cerebrum cutting the capsule and callosum.
Many fibers of the internal capsule give off branches (col laterals) which pass through the corpus callosum to the oppo site hemisphere; other fibers may be traced entire through the same course to the cortex of the opposite side. A bundle of thalamic fibers has been so traced (Hamilton).
The superior lamina of the internal capsule, proceeding out ward and upward into the hemisphere, intermingles with the corpus callosum and enters into the corona radiata. Together with the caudate nucleus, thalamus and stria terminalis(tenia semicircularis), which lie on its medial surface, it forms the entire lateral boundary of the general cavity of the fore-brain.
Corpus Callosum (Figs. 35, 37, 42 and 54).—The entire roof of the fore-brain cavity, representing the base of the wedge, is formed by the corpus callosum. A part of the anterior bound ary is also fomed by it. The corpus callosum is a thick sheet of fibers four and a half inches broad, from before backward, which joins the hemispheres together. It constitutes the great com missure, being composed chiefly of those medullated cortical axones which end in arborizations about cortical cells of the opposite hemisphere. It contains some fibers which belong to the internal capsule; and, also, collaterals from capsular and association fibers. The corpus callosum is placed nearer to the anterior than the posterior pole of the hemispheres. Separat
ing the hemispheres above, it is seen in the bottom of the longitudinal fissure. It is about an inch in transverse length at the posterior end.
The upper surface is concave from side to side and divided in the median line by a longitudinal raphe (Figs. 37 and 42). Transverse striae are plainly visible. Two longitudinal strim are also found running on either side of the raphe; one next the raphe, the medial longitudinal stria; and the other near the lateral end of the callosum, the lateral longitudinal stria. The medial and lateral longitudinal striae are imbedded in a thin sheet of gray substance, the stratum indusium griseum; alto gether they constitute the gyrus supracallosus. If traced around the posterior border of the callosum, this supracallosal gyrus is found to be continuous with the fasciola cinerea and gyrus subsplenialis and through them with the fascia dentata.
The gyrus supracallosus becomes the gyrus subcallosus (peduncle of corpus callosum) and area parolfactoria after it winds around the anterior border of the corpus callosum. The gyrus subcallosus is continued downward between the lamina terminalis and the posterior parolfactory sulcus to the base of the cerebrum, and then, as gyrus diagonalis, across the anterior perforated substance to the uncus. At the anterior and at the posterior border, the corpus callosum is bent down ward (scroll-like); hence, it is superiorly convex from before backward.
•Its inferior surface forms the roof of the lateral ventricles (Figs. 35 and 37). It is concave antero-posteriorly and near its posterior border is fused with the body of the fornix. An terior to that fusion it is joined to the fornix along the median line by the septum pellucidum.
The posterior border (Fig. 35) is flexed downward from the horizontal about forty-five degrees. Giving passage to the fibers which join the middle and posterior parts of the hemi spheres, the posterior border is the thickest part of the corpus callosum. It is on that account called the pad, or splenium. A large bundle of these splenial fibers arches back toward the medial surface of the occipital lobe; they form the forceps major.