CHAPTER XVI 



THE INTERNAL CONFIGURATION OF THE CEREBRAL HEMISPHERES 



WHEN a horizontal section is made through the cerebral hemisphere at the 

 level of the dorsal border of the corpus callosum the central white substance 

 will be displayed in its maximum extent and will appear as a solid, semioval 

 mass, known as the centrum semiovale (Figs. 174, 175). It will also be apparent 

 that lamellae extend from this central white substance to form the medullary 

 centers of the various convolutions, and that over this entire mass the cortex is 

 spread in an uneven layer, thicker over the summit of a convolution than at 

 the bottom of a sulcus. This medullary substance is composed of three kinds 

 of fibers: (1) fibers from the corpus callosum and other commissures joining the 

 cortex of one hemisphere with that of the other; (2) fibers from the internal cap- 

 sule, uniting the cortex with the thalamus and lower lying centers; and (3) 

 fibers running from one part of the cortex to another within the same hemi- 

 sphere (p. 296). 



The Corpus Callosum. At the bottom of the longitudinal fissure of the 

 cerebrum is a broad white band of commissural fibers, known as the corpus 

 callosum, which connects the neopallium of the two hemispheres. While the 

 medial portion of this commissure is exposed in the floor of the longitudinal 

 fissure, its greater part is concealed in the white center of the hemisphere where 

 its fibers radiate to all parts of the neopallium, forming the radiation of the 

 corpus callosum. When examined in a median sagittal section of the brain the 

 corpus callosum is seen to be arched dorsally and to be related on its ventral 

 surface to the fornix and septum pellucidum (Figs. 84, 158, 170). The latter 

 consists of two thin membranous plates, stretched between the corpus callosum 

 and the fornix and separated by a narrow cleft-like space, the cavum septi 

 pellucidi (Fig. 177). If the septum has been torn away it will be possible to 

 look into the lateral ventricle and see that the corpus callosum forms the roof 

 of a large part of that cavity. At its rostral extremity it curves abruptly toward 

 the base of the brain, forming the genu, and then tapers rapidly to form the 

 rostrum. The latter is triangular in cross-section, with its edge directed toward 

 the anterior commissure to which it is connected by the rostral lamina. The 



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