THE NERVOUS SYSTEM 1385 



Lobes of the Cerebral Hemisphere — Frontal Lobe. — ^This is of 

 large size. On the external surface of the hemisphere it is bounded 

 behind by the fissure of Rolando, and below by the posterior 

 horizontal limb of the fissure of Sylvius. On the inferior surface 

 it is bounded behind by the stem of the fissure of Sylvius. On the 

 internal surface it is bounded by the calloso - marginal fissure. 

 The frontal lobe has three surfaces — external, inferior, and internal. 



External Surface. — ^This surface presents three principal sulci, 

 namely, precentral, superior frontal, and inferior frontal. 



The precentral sulcus is more or less parallel to the fissure of 

 Rolando, the ascending frontal or precentral gyrus inter^'ening 

 between the two. It may be a single fissure, but it more frequently 

 consists of two parts, superior and inferior. The superior part is 

 usually joined above by the superior frontal sulcus. The inferior 



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Fig. 581 . — The Right Cerebral and Cerebellar Hemispheres (External 



Surface). 



Red = Frontal Lobe. Blue = Occipital Lobe. 



Orange = Parietal Lobe. Green = Temporal Lobe. 



Purple = Cerebellar Hemisphere. 



part passes superiorly into the middle frontal gyrus for a short 

 distance in a forward and upward direction. 



The superior and inferior frontal sulci extend forwards from the 

 precentraj sulcus. 



The gyri of the external surface are as foDows : ascending frontal, 

 superior frontal, middle frontal, and inferior frontal. 



The ascending frontal or precentral gyrus is bounded behind 

 by the fissure of Rolando or central sulcus, and in front by the 

 superior and inferior parts of the precentral sulcus. It extends 

 from the supero-mesial border of the hemisphere to a little behind 

 the Sylvian point, which corresponds to the place where the 

 stem of the fissure of Sylvius appears on the external surface of 

 the hemisphere, and divides into its three branches. Below the 

 lower end of the fissure of Rolando it is, as a rule, connected with 

 the ascending parietal or postcentral gyrus by an annectant gyrus. 



