THE CEREBRAL HEMISPHERES 



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The more superficial sections reveal relatively more gray than white substance; 

 deeper sections show a reverse condition, and a section immediately dorsad of the 

 corpus callosum reveals, in each cerebral hemisphere, a very extensive semioval 

 field of white substance, the centrum semiovale, surrounded on all sides by a narrow, 

 convoluted margin of gray substance, the cortex. A close examination of the cut 

 surface, in a fresh and normal brain, shows it to be studded with numerous minute 

 red dots (piuicta vasculosa) produced by the escape of blood from divided blood- 

 vessels. 



The Cortex. The cortex, as revealed in such a section, is not of uniform thickness 

 throughout; different regions show different cortical thicknesses. In general, the 

 cortex is somewhat thicker at the summit of a gyre than in the depths of an 

 adjoining fissure, and it is thicker upon the convex than upon the mesal or basal 

 surfaces. The maximum thickness is observed in the cortex of the central gyres 



FIG. 684. Corpus callosum. (From above.) 



and the insula; the minimum at the frontal and occipital poles, notably the latter. 

 Not only is the cortex not of uniform thickness, but it is not of homogeneous 

 structure as seen with the naked eye. An alternation of gray and white stripes 

 is discernible, particularly in the occipital cortex, where a white band runs parallel 

 with the cortical surface between two gray strata; this white stripe, first described 

 by Gennari and usually bearing his name, is also called the band of Vicq d'Azyr. 



The preponderance of white substance over gray substance in the cerebrum is a human charac- 

 teristic concomitant with the relative increase of the association cortex, in turn demanding a more 

 intricate interconnection of the many nerve cells by a multitude of association neurones. These 



