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INTRODUCTION TO NEUROLOGY 



primary factor in the higher mammals has undoubtedly been 

 the great increase in the superficial area of cortical gray matter 

 without a corresponding enlargement of the skull. 



The human cerebral cortex is somewhat arbitrarily divided 

 into frontal, temporal, parietal, and occipital lobes (Fig. 120). 

 These lobes have no special functional significance, but are dis- 

 tinguished merely for convenience of topographic description. 

 Some of the more important gyri and sulci are named on Figs. 

 52 and 54 (pp. 119 and 121). Between the temporal and frontal 

 lobes and under the lower end of the lateral or Sylvian fissure is 

 a buried convolution, the island of Reil (insula), which is seen in 

 section in Figs. 79 and 80 (pp. 166 and 170) . The cortical lobules 

 which cover the insula are called opercula (Fig. 54, p. 121). 



OCCIPITAL 

 LOBE 



Fig. 120. The lateral aspect of the human brain, illustrating the boundaries 

 of the lobes of the cerebral cortex (cf. Fig. 54). 



The walls of the cerebral hemispheres in the cortical region are 

 very thick, the greater part of this thickness being occupied by 

 white matter composed of nerve-fibers which effect various types 

 of connection with the neurons of the cerebral cortex. The 

 cortex itself is composed of gray matter and is relatively thin, 

 its inner border being marked by a broken line in Figs. 79 and 80. 

 The subcortical white matter contains three chief classes of 

 fibers: (1) Corona radiata fibers which connect the cortex with 

 the brain stem (Figs. 79, 80). Most of these fibers pass through 

 the internal capsule and comprise the sensory and motor pro- 

 jection fibers (pp. 165-169); (2) commissural fibers of the corpus 

 callosum and hippocampal commissure (Figs. 79, 80) ; (3) associ- 

 ation fibers, which connect different parts of the cerebral cortex 



