en. XLVI.] 



LOBES OF THE BRAIN 



665 



and the superior parietal convolution, or parietal lobule, in front of the 

 external parieto-occipital fissure. 



3. The occipital lobe is divided into upper, middle, and lower 

 occipital convolutions by two secondary fissures running across it. 



4. The temporal or temporo-sphenoidal lobe is similarly 

 divided into upper, middle, and lower temporal convolutions by two 

 fissures running parallel to the fissure of Sylvius ; the upper of these 

 fissures is called the parallel fissure. 



5. The Island of Reil is divided into convolutions by the break- 

 ing up of the anterior limb of the Sylvian fissure. 



u o e_r 



FIG. 486. Right cerebral hemisphere, mesial surface. 



Coming now to the mesial surface of the hemisphere (fig. 486), 

 its subdivisions are made evident by cutting through the corpus 

 callosum which unites the hemisphere to its fellow. The sub- 

 division into lobes is nob so apparent here as on the external 

 surface of the hemisphere, so we may pass at once to the con- 

 volutions into which it is broken up by fissures. 



In the middle the corpus callosum is seen cut across ; above it 

 and parallel to its upper border is a fissure called the calloso-marginal 

 fissure which turns up and ends on the surface near the upper end 

 of the fissure of Eolando. The convolution above this is called the 

 marginal convolution, and the one below it the callosal convolution or 

 gyrus fornicatus. The deep fissure below the corpus callosum running 

 from its posterior end forwards and downwards is called the dentate 

 fissure; this forms a projection seen in the interior of the lateral 

 ventricle, and called there the hippocampus major ; it is sometimes 

 called the hippocampal convolution which, together with the gyrus 

 fornicatus above the corpus callosum, constitutes the limbic lobe. 

 Below the dentate fissure is another called the collateral fissure, above 

 which is the uncinate convolution, and below which is the inferior 

 temporal convolution which we have previously seen on the external 

 surface of the hemisphere (see fig. 485). In the occipital region the 



