654 STRUCTURE OF THE CEREBRUM. [CH. XLVI. 



and the anterior limb, which passing into the substance of the 

 hemisphere, forms the Island of Reil. 



2. The fissure of Rolando, running from about the middle of 

 the top of the diagram (fig. 487) downwards and forwards. 



3. The external parieto-occipital fissure (PAR. oc. F) parallel to the 

 fissure of Rolando but more posterior and much shorter ; in some 

 monkeys it is longer (see fig. 486). 



EP. 



C6. 



Fig. 486. Brain of the Orang, f natural size, showing the arrangement of the con- 

 volutions. Sy, fissure of Sylvius ; R , fissure of Rolando ; EP, external parieto-oecipital 

 fissure ; Olf, olfactory lobe ; Cb, cerebellum ; PV, pons Varolii ; MO, medulla 

 oblongata. As contrasted with the human brain, the frontal lobe is short and small 

 relatively, the fissure of Sylvius is oblique, the temporo-sphenoidal lobe very promi- 

 nent, and the external parieto-occipital fissure very well marked. (Gratiolet.) 



These three fissures divide the brain into five lobes : 



1 . The frontal lobe ; in front of the fissure of Rolando. 



2. The parietal lobe ; between the fissure of Rolando and the 

 external parieto-occipital fissure. 



3. The occipital lobe; behind the external parieto-occipital fissure. 



4. The temporal or temporo-sphenoidal lobe ; below the fissure of 

 Sylvius. 



5. The Island of Reil. 



It will be noticed that the names of the lobes correspond to 

 those of the bones of the cranial vault which cover them. There 

 is no exact correspondence between the bones and the lobes, but 

 the precise position of the various convolutions in relation to the 

 surface of the skull is a matter of anatomy, which, in these days 

 of brain-surgery, is of overwhelming importance to the surgeon. 

 The position of a localised disease in the brain can be determined 



