THE TELENCEPHALON. 



1141 



Rolandic fissure. The observations of Mills and of Griinbaum and Sherrington 

 emphasize the predominating importance of the precentral convolution as containing 

 the important cortical motor areas (page 121 1), the backward projection just 

 noted containing the centres controlling the muscles of the upper extremity. 



The superior frontal gyrus lies between the supero-mesial border of the hemi- 

 sphere and the superior frontal sulcus. Since its course corresponds with the upper 

 margin of the hemisphere, it is much longer than the other frontal convolutions on 

 the external surface and reaches the frontal pole. It is continuous with the marginal 

 gyrus, which, in fact, is only its mesial part. Behind, it joins the precentral convolu- 

 tion by a narrow bridge between the upper end of the precentral sulcus and that of a 

 branch from the calloso-marginal fissure. The superior frontal convolution, notwith- 

 standing its meagre width, is frequently imperfectly divided into an upper and a 

 lower part by a series of shallow longitudinal furrows collectively termed \\\^ para- 

 median sulcus. The latter is regarded as a distinctive feature of the human brain, 

 and is found relatively deep and well marked only in the brains of the higher races. 



The middle frontal gyrus, the broadest of the three, extends forward parallel 

 with the upper frontal convolution well towards the frontal pole. It is bounded 



Fig. 986. 



Interior precentral sulcus 



Rolandic fissure 



Ascending limb 



Orbital surface/ 



Horizontal limb' 



Posterior limb 



Portion of lateral surface of left hemisphere, showing pars basalis (B), triangularis {T) and orbitalis ((9) of 

 inferior frontal gyrus, known as Broca's convolution : ST., superior temporal gyrus. 



above and below by the superior and the inferior frontal sulcus and, in man and 

 the anthropoid apes, is almost constantly subdivided into an upper and a lower sub- 

 lower subdivision by the mid-fro?ital sulcus (stilcus frontalis medius). The latter is 

 often broken by annectant gyri into two or more pieces and in front usually bifurcates 

 to form the fronto-7narginal sulcus (sulcus transversus anterior), which runs across 

 the hemisphere a short distance above the superciliary margin. 



The inferior frontal gyrus, the shortest of the three, lies below the inferior 

 frontal sulcus and arches forward and downward around the anterior limbs of the 

 Sylvian fissure. Below and behind it is connected with the lower end of the pre- 

 central convolution by a narrow bridge enclosing the lower end of the inferior pre- 

 central sulcus. By the ascending and horizontal limbs of the Sylvian fissure the 

 inferior frontal gyrus is incompletely divided into three portions — the pars basalis, 

 \\iQ pars triangularis a.nd t\\& pars orbitalis (Fig. g86). The pars basalis (pars 

 opercularis) occupies the posterior part of the convolution and lies between the 

 inferior precentral sulcus and the ascending Sylvian limb. It forms the fore-part of 

 the fronto-parietal operculum and is indented by an inconspicuous although constant 

 furrow, the sulcus diagonalis, which extends obliquely downward and forward across 

 the gyrus for a variable distance. Although usually distinct, the diagonal sulcus 

 may join the inferior precentral (Fig. 986), the inferior frontal or the Sylvian 

 fissure. The pars triangularis is the wedge-shaped tract included between the 

 two limbs of the Sylvian fissure. Its base is directed upward and forward and its 



