664 



STRUCTURE OF THE CEREBRUM 



[CH. XLYI. 



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

 external parieto-occipital fissure. 



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



4. The 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 very accurately, as 

 we shall see later, by the symptoms exhibited by the patient, and it 

 would be obviously inconvenient to the patient if the surgeon was 

 unable to trephine over the exact spot under which the diseased con- 

 volution lies, but had to make a number of exploratory holes to find 

 out where he was. 



Each lobe is divided into convolutions by secondary fissures. 



1. The frontal lobe is divided by the central frontal or prefrontal 

 sulcus, which runs upwards parallel to the fissure of Eolando, and two 



O N T A I. 



PAR. OC. FA 



FIG. 485. Bight cerebral hemisphere, outer surface. 



transverse frontal sulci, upper and lower, into four convolutions ; 

 namely, the ascending frontal convolution in front of the fissure of 

 Eolando, and three transverse frontal convolutions, upper, middle, and 

 lower, which run outwards and forwards from it. 



2. The parietal lobe has one important secondary sulcus, at first 

 running parallel to the fissure of Eolando and then turning back 

 parallel to the margin of the brain. It is called the intra-parietal 

 sulcus. The lobe is thus divided into the ascending parietal convolu- 

 tion behind the fissure of Eolando, the supra-marginal convolution 

 between the intra-parietal sulcus, and the fissure of Sylvius; the 

 angular convolution which turns round the end of the Sylvian fissure, 



