THE NERVOUS SYSTEM 



1387 



The inferior frontal gyrus lies below the inferior frontal sulcus, 

 and in front of the lower part of the precentral sulcus. The anterior 

 horizontal and the ascending hmbs of the fissure of Sylvius enter 

 it and subdivide it into three parts, namely, pars orbitaUs, pars 

 triangularis, and pars basilaris. The pars orhitcdis lies below the 

 anterior horizontal limb of the fissure of Sylvius ; the -pars triangu- 

 laris is situated between the anterior horizontal and the ascending 

 limbs of the fissure of Sylvius ; and the pars basilaris is placed between 

 the ascending limb of the fissure of Sylvius and the lower part of the 

 precentral sulcus. The inferior frontal gyrus is connected posteriorly 

 with the lower end of the ascending 

 frontal or precentral gyrus by an 

 annectant gyrus. 



The inferior frontal gyms of the 

 left side is specially important, 

 inasmuch as the speech-centre is 

 assigned to the portion of it which 

 forms the pars triangularis (le cap 

 of Broca). 



Inferior or Orbital Surface of the 

 Frontal Lobe. — ^This surface pre- 

 sents two sulci, olfactory and 

 orbital. 



The olfactory sulcus is parallel to 

 the mesial border, from which it 

 is separated by the gyms rectus. 

 It lodges the olfactory tract and 

 olfactory bulb. The orbital sulcus 

 (triradiate sulcus of Turner) is of 

 very variable form, but, as a rule, 

 bears some resemblance to the 

 letter M. It has therefore three 

 limbs — inner, outer, and transverse. 

 The inner limb is separated from 

 the olfactory sulcus by the internal 

 orbital gyrus. The oiiter limb is 

 curved, and has external to it the 

 orbital part of the inferior frontal 

 gyrus. The transverse limb passes 

 in a more or less curved manner 

 between the inner and outer limbs. 



The gyri of the orbital surface 

 are as follows : gyrus rectus, in- 

 ternal orbital gyrus, anterior orbital 

 gyrus, external orbital gyrus, and 

 posterior orbital gyrus. 



The gyrus rectus lies between the olfactory sulcus and the mesial 

 border. The internal orbital gyrus is placed between the olfactory 

 sulcus and the inner limb of the orbital sulcus. The external 



Fig. 583. — The Inferior Surface 

 OF THE Left Cerebral Hemi- 

 sphere, SHOWING THE GYRI 



AND Sulci. 



