HEMISPHERES OF THE BRAIN. 



649 



sulcus, the sulcus frontalis mesialis of Cunningham, which, however, is frequently 

 interrupted and broken into several parts by bridging convolutions. 



The middle frontal convolution is situated between the superior and inferior 

 frontal sulci, and extends from the precentral sulcus on to the orbital surface of 

 the lobe, where it forms the anterior orbital convolution. The middle frontal 

 convolution is frequently subdivided into two by a sagittally directed sulcus, the 

 sulcus frontalis medius of Eberstaller. 



The inferior frontal convolution is situated below the inferior frontal sulcus, 

 and extends forward from the lower part of the precentral sulcus, on to the under 

 surface of the lobe, where it forms the posterior orbital convolution. The inferior 

 frontal convolution is subdivided by the anterior and ascending limbs of the 

 fissure of Sylvius into three parts, viz. : (1) anterior or pars orbitalis, below the 



QYRUS 

 RECTUS 



ANTERIOR 



PERFORATED 



SPACE 



FIG. 347. Convolutions and sulci on the under surface of the anterior lobe. 



anterior limb of the fissure; (2) middle or pars triangularis ( kk cap " of Broca), 

 between the two limbs ; and (3) posterior or pars basalis, behind the ascending 

 limb. 



The left inferior frontal convolution is, as a rule, more highly developed than 

 the right, and is named the convolution of Broca, from the fact that in 1861 

 Broca discovered that it was the centre for language. 



The under surface of the frontal lobe rests on the orbital plate of the frontal 

 bone, and is sometimes named the orbital lobe (Fig. 347). It is divided into three 

 convolutions by a well-marked sulcus, the orbital or tri-radiate sulcus. These are 

 named, from their position, the internal, anterior, and posterior orbital convolutions, 

 and are the continuations respectively of the superior, middle, and inferior frontal 

 convolutions of the external surface. The internal orbital convolution presents a 

 well-marked antero-posterior groove or sulcus, the olfactory sulcus, for the olfactory 



