148 



THE CEREBRAL HEMISPHERES. 



posterior end by an annectent gyrus with the lower end of the precentral, while at 

 its anterior end it passes round to the orbital surface of the lobe. 



This gyrus is usually regarded as being bounded posteriorly by the inferior 

 precentral sulcus, but it is,. for many reasons, more convenient to regard the trans- 

 verse precentral as the posterior limit. 



It is occasionally subdivided into two by a longitudinal sulcus concentric with the inferior 

 frontal (Giacomini). 



The pars basilaris is subdivided into two parts (anterior and posterior divisions) by an 

 oblique fissure, the sulcus diagonalis of Eberstaller (figs. 101, 106. d). This is usually distinct, 

 but sometimes joins the inferior precentral (fig. 106, right side), the inferior frontal (fig. 106, 

 left side), or even the Sylvian fissure. The last is the case in fig. 101. The pars triangularis 

 is also often scored by secondary sulci (figs. 101, 106). 



According to most modern authorities the third frontal gyrus is almost or quite rudimentary 

 in all primates excepting man. Cunningham points out, however, that although it is true 

 that the opercular part of this convolution is not present in the apes, the same remark does 

 not apply to the non-opercular part, and he believes that the inferior frontal sulcus, which is 



Fig. 107. BRAIN OF MA- 

 CACQUE MONKEY. Natural 

 size. (E. A. S.) 



Sulci. A.-SJJ, "Affen- 

 spalte ; " i. -p, intraparietal ; 

 i.-p.s i superior part of intra- 

 parietal ; Bo., Kolandic ; 

 Sy. j), Sylvian, posterior limb; 

 f lt superior frontal; .?.?'., sul- 

 cus rectus (inferior frontal) ; 

 p. c. i, precentral inferior ; 

 p.c.i.a., its anterior ram us ; 

 p.c. tr., transverse precen- 

 tral ; 1? parallel. 



regarded by most authorities as not represented in the apes, is, in fact, represented in them by the 

 well-marked sagittal furrow (irulciis rectvg quadrumanorum, fig. 107, #.r.~) which runs forwards 

 from near the angle of the inferior precentral sulcus to the apex of the hemisphere. This view, 

 which was that taken by Gratiolet (Memoire sur les plis cerebraux de 1'homme et des primates, 

 1854), gives a well-developed third frontal gyrus to the ape brain, whereas if the sulcus in 

 question be regarded as the superior frontal (Schwalbe, Mingazzini), or as the sulcus frontalis 

 medius (Eberstaller, Herve) of the human brain, the sulcus frontalis inferior must be absent 

 in the ape, or at most represented by a small fronto-orbital sulcus which is sometimes 

 present at the lowermost limit of the lobe (see fig. 107). But the superior frontal sulcus of 

 man is represented in the monkey by one or more deep indentations of the cortex lying in a 

 sagittal line near the margin and sometimes partly confluent (fig. 107,/ t ), and in some apes 

 there is also a distinct mid-frontal sulcus besides the sulcus rectus : moreover, the study of 

 the development of the inferior frontal sulcus in man is also in favour of Gratiolet's view. 

 For a full discussion of this question the student is referred to Prof. D. J. Cunningham's 

 memoir, " The Surface Anatomy of the Primate Cerebrum," Dublin, 1892. 



The anterior central, precentral, or ascending frontal gyrus (fig. 101, 

 A.F.) lies between the fissure of Rolando and the precentral sulcus, and extends from 

 the superior margin of the hemisphere (where it is continuous with the posterior 

 part of the marginal gyrus on the mesial surface) to the Sylvian fissure. It is 

 narrowest in the middle, enlarging somewhat above and below. It is usually con- 

 nected, as just mentioned, with each of- the three longitudinal frontal convolutions 

 by an annectent gyrus, the precentral sulcus being thereby considerably interrupted. 

 There is a similar connection by a curved annectent gyrus below the fissure of 

 Rolando, with the ascending parietal convolution. 



SULCI AND GYKI OF THE MESIAL SURFACE. On the mesial surface of the 

 hemisphere there is only one convolution which is considered to belong to the frontal 

 lobe, viz., the marginal gyrus (fig. 102, FJ. This, as above explained, is continuous 

 over the upper border with the superior longitudinal of the external surface. It is 

 separated from the callosal or fornicate gyrus of the limbic lobe by the calloso-marginal 



