THE FORE-BRAIN 



531 



Each hemisphere presents an outer convex surface lying in 

 the vault of the skull; a Hat inner or mesial surface forming one 

 side (if the longitudinal sulcus ; and an irregular lower surface in 

 which there is the deep fissure of Sylvius. As shown by Figs. 

 264, 265, 266, all three surfaces 'of the cerebral hemispheres 

 present numerous fissures or sulci, marking out as many smooth 

 and winding projections, the convolutions or gyri. The surface 

 of the brain is enormously increased by this folding into sulci 

 and irvri. The extent of the infolded surface is estimated at 



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double that of the visible surface. 



The membranes that envelop the brain resemble those of the 



Fir;. 204.- External aspect of left cerebral hemisphere. The names of the gyri and lobules are 

 marked in Roman type ; those of the sulci and fissures in italics. 



spinal cord in structure. The pia mater, which is very rich in 

 vessels, dips down into the bottom of the sulci, while the arachnoid 

 passes from one convolution to the next without penetrating between 

 them ; the whole floats in the sac of the dura mater. 



The primary sulci, which are seen in the foetal human brain 

 and in adult apes, must be distinguished from the secondary sulci ; 

 the former divide the hemispheres into lobes, the latter subdivide 

 the lobes into gyri or convolutions. 



The primary sulci are the Sylvian fissure (fissura cerebri 

 lateralis), the sulcus of Rolando (sulcus centralis), and the parieto- 

 occipital sulcus. The lobes formed by these sulci are the frontal, 

 temporal, parietal, occipital, and central (or island of Reil). The 

 convolutions of each lobe are shown with their names in the three 

 diagrammatic figures. 



