THE THIRD FRONTAL GYRUS. 



147 



gyri. They are separated from one another by the upper and lower longitudinal 

 frontal sulci, and from the fourth or ascending frontal gyrus by the precentral 

 sulcus, which runs parallel to the fissure of Rolando. 



The first frontal gyrus (FJ, which is much the longest, runs parallel with the 

 upper margin of the hemisphere, over which it is continuous with the marginal con- 

 volution of the mesial surface, the two, in fact, really forming a single convolution to 

 which the name first frontal is frequently applied. This convolution reaches the frontal 

 pole of the hemisphere in front ; behind it is partly continued into the upper end 

 of the precentral or ascending frontal gyrus, partly separated from that gyrus by the 

 upper end of the precentral sulcus ; below it is marked off from the middle frontal 

 by the irregular and somewhat interrupted superior frontal sulcus. It is sometimes 

 subdivided by a longitudinally coursing sulcus (paramesial, s. frontalis mesialis of 

 Cunningham) into two parts, but less often than the second gyrus. This subdivision 

 is rarely found in the brain of the negro. 



The second frontal gyrus (F. 2 ), runs below and parallel to the first. It is 

 separated from the precentral by the precentral sulcus, the course of which is, 

 however, here usually interrupted by a well-marked annectent gyrus. The inferior 

 frontal sulcus separates it from the third gyrus. It is also often subdivided by a 

 longitudinally coursing sulcus (midfrontal sulcus) into two parts, an upper or lower, 

 which are sometimes described as distinct convolutions. 



The third frontal gyrus (^ 3 ) the smallest of the three, is curved around both 

 the anterior and the ascending limbs of the Sylvian fissure, which deeply indent the 



Left side. 



Right side. 



Fig. 106. VIEWS OF THE THIRD FRONTAL GYRUS OF THE RIGHT AND LEFT HEMISPHERES OF THE SAME 



BRAIN. NATURAL SIZE. FROM PHOTOGRAPHS. (R. A. S. ) 



/ 2 , inferior frontal sulcus ; p.c. inf., lower end of inferior precentral, very short, and not lettered 

 on the left side, and continuous on the right side with, d, diagonal sulcus (on the left side d joins /,) ; 

 p.c. tr, transverse precentral ; Sij. a, anterior limb of Sylvian fissure ; x, ramus ascendens, y, ramus 

 horizontalis ; a, posterior part of third frontal (divided by the diagonal sulcus into two parts, viz., pars 

 basilaris behind, and pars ascendens in front) ; b, middle part of third frontal or pars triangularis, 

 partly subdivided on the left side by secondary sulci ; c, orbital part of third frontal. 



gyrus, and subdivide it into three parts, anterior (pars orbitalis), middle (pars trian- 

 gularis), and posterior (pars basilaris). It is more developed on the left side of the 

 brain than on the right correspondingly with the localization of the speech-centre 

 on the left side (Broca). This increased development chiefly affects the triangular 

 middle part which lies between the anterior and ascending limbs of the Sylvian 

 fissure (" cap " of Broca). "When well developed, the triangular part entirely separates 

 these two limbs (tig. 106, left side), when less marked, they are confluent below (fig. 

 106, right side); if the triangular part is abs'ent they are entirely confluent (see also 

 fig. 104 and p. 142). This inferior frontal gyrus is connected at its lower and 



L 2 



