INTERIOR SURFACE OF THE FORE-BRAIN. 99 



the diverging internal capsules, which, within the hemispheres, 

 decussate with the lateral extremities of the corpus callosum. 

 The two structures most necessary to an understanding of the 

 cavities and interior surface of the fore-brain, therefore, are the 

 internal capsule and the corpus callosum. 



Internal Capsule. (Capsula Internd). 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 45). 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. 33). 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 hi its continuity, 

 the hiatus Sylvii; otherwise the funnel is complete. As the inter- 

 nal 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 capsulate) the 

 lentiform nucleus. The lentiform nucleus is separated externally 

 from the claustrum by a thin layer of fibers called the external 

 capsule. 



The internal capsule is directed obliquely outward and upward 

 and is flattened from above downward. It has, therefore, a 

 superior and an inferior lamina, which, posteriorly, are continuous 

 with each other but, anteriorly, are separated by the hiatus Sylvii. 



The inferior lamina (or inferior ramus, as seen in sagittal 

 section) is thick behind but bevels down to a sharp edge anteriorly 

 (Figs. 32, 33, and 70). In front it presents a free border. Its 

 fibers pass outward beneath the lentiform nucleus and, after 

 winding over the inferior horn of the lateral ventricle, terminate 

 in the parietal, the insular and temporal lobes. 



