THE BRAIX AXD ITU MEMBRANES. 



775 



brain and the fissure of Rolando, and runs downward and forward for somewhat 

 less than an inch. It separates the parietal and occipital lobes. 



These three fissures divide the external surface of the hemisphere into five 

 lobes the frontal, the parietal, the occipital, the temporal, and the central or 

 island of JieiL 



The frontal lobe is that portion of the brain which is situated in front of the 

 fissure of Rolando and above the horizontal limb of the fissure of Sylvius. Its 

 under surface rests on the orbital plate of the frontal bone, and is termed the 

 orbital lobe. 



The outer surface of the frontal lobe presents three sulci, which divide it into 

 four primary convolutions : 1. The precentral sulcus runs upward through this 

 lobe, parallel to the fissure of Rolando. It may be interrupted by annectant 



End of calloso- 

 marginal fissure. 



Ascending fissure 

 of Sylvius. 



FIG. 466. Convolutions and fissures of the outer surface of the cerebral hemisphere. 



gyri. It divides off a convolution which lies between it and the fissure of Rolando, 

 and which is called the ascending frontal convolution. 2 and 3. From it two sulci, 

 the superior and inferior frontal sub-i, run forward and downward, and divide the 

 remainder of the outer surface of the lobe namely, that part in front of the pre- 

 central sulcus into three principal convolutions, named, respectively, the superior, 

 middle, and inferior frontal convolutions (or "lobes"). 



The under surface of the frontal lobe, which rests on the orbital plate of the 

 frontal bone, is named the orbital lobe (Fig. 467). This surface of the frontal 

 lobe is divided into three convolutions by a well-marked sulcus, the orbital sub-it*. 

 These are named, from their positions, the iiitcnmL anterior, and posterior orbital 

 '"//'olutions, and are the continuations respectively of the superior, middle, and 

 inferior frontal convolutions. The internal orbital convolution presents or is 

 subdivided by a well-marked groove or sulcus (olfactory sulcus) for the olfactory 

 tract. 



The ascending frontal convolution is a simple convolution, bounded in front by 

 the precentral sulcus, behind by the fissure of Rolando, and extending from the 



