CH. XLVI.] 



LOBES OF THE BRAIN. 



655 



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 convolution 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 pre- 

 frontal sulcus which runs upwards parallel to the fissure of Rolando, 

 and two transverse frontal sulci, upper and lower, into four convo- 

 lutions ; 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 mnning parallel to the fissure of Rolando and then turning 

 back parallel to the margin of the brain. It is called the 



Fig. 487. Right cerebral hemisphere, outer surface. 



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

 parietal convolution 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. 



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

 occipital convolutions by two secondary fissures running across it. 



4. The temporal 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 Beil is divided into convolutions by the 

 breaking up of the anterior limb of the Sylvian fissure. 



Coming now to the mesial surface of the hemisphere (fig. 488), 

 its subdivisions are made evident by cutting through the corpus 



