CH. XLVIIL] 



THE CEREBRAL CONVOLUTIONS 



693 



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 precentral 

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

 transverse frontal sulci, upper and low^er, into four convolutions; 

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

 Rolando, 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 Rolando 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 Rolando, 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, 

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

 external parieto-occipital fissure. 



UQBC 



Fio. 430. Right cerebral hemisphere, mesial surface. 



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. 



