THE CEREBRUM 469 



are termed the pars basilaris, the pars triangularis, and the 

 pars orbkalis. 



The pars basilaris (Fig. 186) is that part which lies be- 

 tween the vertical limb of the inferior praecentral sulcus and 

 the anterior ascending ramus of the lateral fissure. It forms 

 the anterior portion of the fronto-parietal operculum, and it 

 is traversed in an oblique direction by a shallow furrow, 

 termed the sulcus diagonalis. 



The pars triangularis (Fig. 186) is simply another name 

 for the frontal operculum. It is triangular in form, and lies 

 between the anterior ascending and anterior horizontal rami 

 of the lateral fissure. 



The pars orbitalis (Fig. 186) is placed below the anterior 

 horizontal ramus of the lateral fissure. 



Medial Surface of the Frontal Lobe. On this aspect of 

 the frontal lobe is the elongated, more or less continuous, 

 medial part of the superior frontal gyrus (O.T. marginal 

 gyrus). It lies between the supero-medial border of the 

 hemisphere and the sulcus cinguli (Fig. 187). 



In the anterior part of this gyrus one or two curved sulci are usually 

 present. These are termed the sulci rostrales. 



The posterior end of the medial surface of the superior 

 frontal gyrus is more or less completely cut off from the 

 anterior part. It forms part of the paracentral lobule, and 

 lies anterior to the upper end of the central sulcus (Fig. 187). 



Orbital Surface of the Frontal Lobe. On this aspect of 

 the frontal lobe there are two sulci viz. the olfactory and 

 the orbital. 



Sulcus Olfactorius. The olfactory sulcus (Fig. 188) is a 

 straight furrow which runs parallel to the medial orbital 

 border of the hemisphere. It is occupied by the olfactory 

 tract and bulb, and it cuts off a narrow strip of the orbital 

 surface close to the medial border which receives the name 

 of gyrus rectus (Fig. 188). 



The orbital sulcus is a compound furrow which assumes 

 many different forms. Most frequently it takes the shape of 

 the letter H, of which the three component parts are a 

 lateral limb, a medial limb, and a transverse limb. 



The lateral limb curves round the orbital part of -the 

 inferior frontal gyrus, and limits it medially. The medial 

 limb marks off a convolution between itself and the olfactory 



