CONVOLUTIONS OF THE FRONTAL LOBE. 749 



but the whereabouts of each is indicated. (Compare figs. 270 and 

 272). 



The opercula are called (1) orbital, (2) frontal, (3) fronto- 

 parietal, and (4) temporal. 



1. The orbital operculum is the back part of the posterior orbital, 

 orbital convolution (P.O., fig. 272) of the frontal lobe which conceals 

 the front part of the island. 



2. The frontal optrcHhirtt is the overhanging piece of the frontal frontal, 

 lob..- between the anterior and ascending limbs of the Sylvian 

 fissure. 



3. The froiito-piirii-tI operculum is the part belonging to the fronto- 

 frontul and parietal lobes that overlaps the island behind the P anetal > 

 ascending limb of the fissure of Sylvius. 



4. The temporal operculum is the projecting anterior part of temporal, 

 the temporal lobe. 



SULCI AND CONVOLUTIONS OF THE FRONTAL LOBE. On the Ollter 



surface of the frontal lobe there are four convolutions, separated by 

 three sulci. 



The prcecentral sulcus (fig. 270, pr c) is placed in front of, and Frontal 

 nearly parallel to, the lower half of the snlcus of Rolando. From g^USsS 

 it the inferior frontal sulcn.s (if) runs forwards and downwards, and two 

 towards the orbital surface of the lobe. Above this, the superior tudfnal. 

 frontal sulcus (.*/), which is often interrupted once or twice by cross 

 gyri, takes a similar course. 



The ascending frontal convolution (AF) is simple, and forms the Frontal con- 

 hindmost part of the frontal lobe, extending from the upper margin J'JjJnsverse 

 of the hemisphere to the Sylvian fissure, along the front of the 

 furrow of Rolando. From its fore part the three longitudinal 

 convolutions of this lobe take their origin. 



The superior frontal convolution (SF) is longer and broader than superior, 

 the others, and is commonly subdivided by a special sulcus 

 paramedialis into secondary gyri. It lies between the margin of 

 the hemisphere and the upper frontal sulcus. 



The middle frontal convolution (MF) runs from the ascending middle, 

 frontal to the lower margin of the lobe, between the upper and 

 lower frontal sulci. Like the superior frontal convolution, it is 

 also often subdivided into upper and lower parts by a sulcus (sulcus 

 front alis rnedius, Eberstaller) running along it. 



The inferior frontal convolution (IF) is the smallest of all. Spring- and inferior 

 ing from the lower end of the ascending frontal convolution, it arches [udfnal 

 round the ascending and anterior branches of the Sylvian fissure, 

 and passes into the posterior orbital gyrus. It is sometimes described Three parts 

 as consisting of three parts : inferior : 



1. pars basilaris, between the ascending limb of the fissure of pars 

 Sylvius and the inferior part of the praecentral sulcus. 



2. pars trianyularis, between the ascending and anterior limbs of pars 

 the Sylvian fissure, being only another name for the frontal 

 operculum ; and 



3. the pars orbitali-s. the part below the anterior limb of the pars 



j. c fi. orbitalis. 



fissure of Sylvius 



