496 



THE NERVOUS SYSTEM. 



callosum. The surface presents depressions, fissures and sulci, 

 and prominences, convolutions or gyri. The external portion of 

 the hemispheres is gray nervous matter about 3 mm. in thickness, 

 beneath which is white matter. The fissures are not numerous, 

 but are quite constant ; they are folds of the brain-matter both 

 gray and white. The sulci are depressions of the gray matter 

 alone ; they are very numerous and inconstant. As gray matter 

 is present on both sides of the fissures and sulci, this arrangement 

 permits of a larger amount of gray matter than could exist were it 

 only upon the surface of the convolutions. In a brain, therefore, 

 where the sulci are deep and numerous the amount of gray matter 



FIG. 287. Outer surface of the left hemisphere : A, anterior central or ascending 

 frontal convolution ; B, posterior central or ascending parietal convolution ; c, sulcus 

 centralis or fissure of Eolando ; cm, termination of the callosomarginal fissure ; F, 

 frontal lobe ; F\, superior, l^Vniddle, and Fz, inferior frontal convolution ; /i, supe- 

 rior, and /2, inferior frontal sulcus ; /a, sulciu prsecentralis ; ip, sulcus intraparietalis ; 

 0, occipital lobe ; Oi, first, Oz, second, Os, third occipital convolutions ; 01, sulcus 

 occipitalis transversus ; 02, sulcus occipitalis longitudinalis inferior ; P, parietal lobe ; 

 po, parieto-occipital fissure ; Pi, superior parietal or posteroparietal lobule ; Pz, infe- 

 rior parietal lobule, viz., P 2 , gyrus supramarginalis ; P{, gyrus angularis ; S, fissure 

 of Sylvius ; S', horizontal, S", ascending ramus of the same ; T, temporosphenoidal 

 lobe: Tij first, Tz, second, Ts, third temporosphenoidal convolutions; ti, first, tz, 

 second temporosphenoidal fissures. 



exceeds that in a brain where they are more shallow and less 

 abundant. This gray matter is the cortical substance. 



Fissures of the Cerebrum. The fissures serve as landmarks 

 in the description of the diiferent parts of the hemispheres. 

 Besides the great longitudinal fissure, there are (1) the fissure of 

 Sylvius ; (2) that of Rolando ; and (3) the parieto-occipital fissure. 

 These fissures divide each hemisphere into 5 lobes. 



(1) The fissure of Sylvius is, next to the great longitudinal 

 fissure, the most important. It is found in all animals whose 

 brains have any fissures. It exists in the human brain in the 



