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Ventricles and Gross Structure of the Fore-Brain

fibers, inferior, ventricle, nucleus, lamina and internal

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VENTRICLES AND GROSS STRUCTURE OF THE FORE-BRAIN The ventricles of the fore-brain comprise the third ventricle, the aula, the interventricular foramina and the two lateral ventricles. The third is the ventricle of the inter-brain. It communicates posteriorly with the fourth ventricle through the cerebral aqueduct; anteriorly it is in direct continuity with the aula, which is the median ventricle of the end-brain. The aula opens on each side into the ventricle of the cerebral hemisphere through the interventricular foramen of Monro. Being out side the median plane, the ventricles of the hemispheres are the lateral ventricles. The lateral ventricles, excepting the inferior horns, occupy a level superior to the aula and third ventricle. In a frontal section of the fore-brain, the ventricles form a T-like figure; the third ventricle and aula constitute the stem, the two lateral ventricles form its branches. These ventricles are roofed over by the white corpus calloum, which stretches from one hemisphere to the other; and the converging internal capsules and the basal ganglia form most of the floor and lateral walls. The extent and relations of these ventricles will be made clear by reference to the embryonic brain (Fig. 17) and by the study of the gross structures revealed in the sections of the fore-brain.

Internal Capsule (Capsula interna).—Looking at the base of the brain we see two broad bands of nerve fibers, the bases pedunculi, issue from the cerebral hemispheres under cover of the optic tracts and, converging downward and backward, dis appear in the pons (Figs. 21 and 57). Traced in their reverse direction, the fibers of each basis pedunculi enter the hemisphere of the cerebrum and are reinforced by a great number of addi tional fibers from the thalamus. The fibers then radiate toward the cerebral cortex in the form of a hollow cone or funnel. This funnel-like group of fibers is the internal capsule (Fig. 4o). The bell of the funnel opens upward and outward and contains the lentiform nucleus; its solid spout, directed toward the pons and medially, is the basis pedunculi. Antero-inferiorly the fibers

in the bell of the funnel diverge to opposite sides of the fissura cerebri lateralis (Sylvii) and produce a break in its continuity, the hiatus capsulce; otherwise the funnel is complete. As the internal capsule proceeds into the hemisphere, it impales the corpus striatum in such manner as to place the caudate nucleus upon its circumference and to inclose within its walls (to cap sulate) the lentiform nucleus. The lentiform nucleus is sepa rated externally from the claustrum by a thin layer of fibers called the external capsule.

The internal capsule is directed obliquely outward and up ward and is flattened from above downward. It has, therefore, a superior and an inferior lamina which posteriorly, are con tinuous with each other, but anteriorly are separated by the hiatus capsulx. In horizontal section the superior lamina presents a sharp angle, the genu, directed toward the median plane, which divides the lamina into an occipital and a frontal part.

The inferior lamina (or inferior ramus as seen in sagittal section) is thick behind but bevels down to a sharp edge anteriorly (Figs. 39, 4o, and 93). In front it presents a free border. Its fibers pass beneath the lentiform nucleus. The inferior lamina of the internal capsule is made up of three composite funiculi, the ventral stalk of the thalamus, the strio fugal fibers and the upper segment of the temporo-pontal tract. In the anterior part of the lamina, in front of a transverse plane cutting the mammillary bodies, the ventral stalk of the thalamus, or ansa peduncularis, and the strio-fugal tracts are located. The ventral stalk is divided by the nucleus inter ansalis into a superior and an inferior stratum. The superior stratum is the ansa lenticularis, which has both terminal and genetic relations with the lentiform nucleus; the inferior stratum is called the inferior peduncle of the thalamus. Both strata of the ventral stalk contain afferent and efferent fibers.

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