THE CEREBRUM 407 



lateral fissure and the inferior precentral sulcus, and is continu- 

 ous, posteriorly, with the anterior central gyrus. The 

 middle part, called also the pars triangularis, lies between 

 the anterior ascending and the anterior horizontal rami of the 

 lateral fissure. The anterior part, which has been termed the 

 pars orbitalis, is placed below and anterior to the anterior 

 horizontal ramus of the lateral fissure, and it is continuous with 

 the posterior orbital gyrus of the inferior surface (Fig. 152). 



Additional Sulci of the Supero-lateral Surface of the Frontal Lobe. 

 There are four fairly constant sulci on the supero-lateral surface of the 

 frontal lobe, besides those which intervene between its principal gyri ; they 

 are the paramedial, the middle, the fronto-marginal, and the diagonal. 



The paramedial siilcus is either a continuous sulcus which lies between 

 the supero-medial border and the superior frontal sulcus, and separates the 

 superior frontal gyrus into upper and lower parts, or it is represented by a 

 series of separate depressions (Fig. 137). 



The middle frontal sulcus separates the middle frontal gyrus into 

 upper and lower parts. It terminates anteriorly, close to the superciliary 

 border of the hemisphere, in a transversely placed limb called the fronto- 

 marginal sulcus (Fig. 152). 



The diagonal suiciis lies parallel with the inferior part of the inferior 

 precentral sulcus, in the posterior part of the inferior frontal gyrus, inter- 

 vening between two structurally different parts of that portion of the gyrus. 



The Inferior Surface of the Frontal Lobe forms the orbital 

 or anterior part of the inferior surface of the hemisphere. 

 It rests upon the roof of the orbit and the roof of the 

 nose. It is bounded, anteriorly and laterally, by the super- 

 ciliary border, and, medially, by the medial orbital border. 

 Posteriorly, in the lateral part of its extent, its boundary is 

 the stem of the lateral fissure, but, more medially, it is 

 separated from the anterior perforated substance by a sulcus 

 which has been named the fissura prima. 



The Sulci of the Inferior Surface of the Frontal Lobe are the 

 olfactory sulcus and the orbital sulci (Fig. 138). 



Sulcus Olfactorius. The olfactory sulcus lies parallel with, 

 and a short distance from, the medial orbital border. It 

 lodges the olfactory bulb. and the olfactory tract. 



Sulci Orbitales. The orbital sulci are irregular in arrange- 

 ment, but generally they assume, collectively, a somewhat 

 H-shaped form, the transverse bar of the H being at right 

 angles to the long axis of the hemisphere. 



The Gyri of the Inferior Surface of the Frontal Lobe are the 

 gyrus rectus and the orbital gyri. 



Gyms Rectus. The gyrus rectus is the district medial to 

 the olfactory sulcus, and it extends from the frontal pole to 



