694 



STRUCTURE OF THE CEREBRUM 



[CH. XLVIII. 



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

 ing up of the anterior limb of the Sylvian fissure. 



Coming now to the mesial surface of the hemisphere (fig. 430), 

 its subdivisions are made evident by cutting through the corpus 

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

 division into lobes is not 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 ; the hippocampal 

 convolution, together with the gyrus fornicatus above the corpus 

 callosum, constitutes the limbic lobe. Bolow 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. 429). In the occipital region the internal parieto- 

 occipital fissure, which is a continuation of the external parieto-occipital 

 fissure, passes downwards and forwards till it meets the calcarine 

 fissure, which is a primary fissure ; these two enclose between them a 

 wedge-shaped piece of brain called the 

 cuneus or cuneate lobule; the square 

 piece above it is called the precuneus 

 or quadrilateral lobule. 



The only convolutions now left are 

 those which are placed on the surface 

 of the frontal lobe that rests on the 

 orbital plate of the frontal bone ; they 

 are shown in fig. 395, 2 % 2" (p. 642), 

 and may be seen diagrammatically in 

 fig. 431, the end of the temporal lobe 

 being cut off to expose the convolu- 

 tions of the central lobe or Island of 

 Eeil. 



Along the edge is the continuation 

 of the marginal convolution (M); next comes the olfactory sulcus 

 (o), in which the olfactory tract and bulb lie; then the triradiate 

 orbital sulcus (o.s.), which divides the rest of this surface into three 

 convolutions. 



A.P.S 



FIG. 431. Orbital surface of frontal lobe. 



M, marginal convolution. 



0, olfactory sulcus. 

 O.S., orbital sulcus. 



1, Island of Reil. 



S.a., anterior limb of Sylvian fissure. 

 S.p., posterior limb of Sylvian fissure. 

 A.P.S., anterior perforated spot. 



