1 142 HUMAN ANATOMY. 



apex towards the Sylvian point. The pars orbitalis hes below the horizontal limb 

 and is continued around the margin of the hemisphere onto the orbital surface of 

 the frontal lobe. It is evident, from the description of the boundaries of the Sylvian 

 fissure already given (page 1137), that the preceding subdivisions of the inferior 

 frontal gyrus correspond with certain of the opercula — the pars basalis with the 

 anterior part of the fronto-parietal, the pars triangularis with the frontal and the 

 pars orbitalis with the orbital operculum. The posterior extremity of the inferior 

 frontal gyrus on the left side is known as Broca s convolution and has long been 

 regarded as the centre for the movements for articulate speech, although the accuracy 

 of this view has been questioned. According to Marie, Broca' s convolution has 

 no relation with speech, a conclusion, however, so far not convincingly supported. 

 The convolution is sometimes better de^'eloped on the left than the right side of 

 the brain, the pars triangularis particularly being increased. As previously noted, 

 the development of this wedge — the frontal operculum — bears a direct relation to 

 the degree of independence of the two anterior limbs of the Sylvian fissure. 



The mesial snrface of the fro7ital lobe {Y\g. 987), includes only one convolution, 

 the marginal gyrus, which lies between the dorso-mesial margin of the hemisphere 

 and the calloso-marginal sulcus (page 11 39), and by the latter is separated from the 

 limbic lobe. It is — 3-shaped and directly continuous with the superior frontal gyrus 

 above and with the gyrus rectus on the orbital surface below. Its posterior end is 

 almost completely cut off from the rest of the gyrus by an ascending limb (sulcus para- 

 centralis) from the calloso-marginal sulcus, the portion so isolated forming the front 

 part of the paracentral lobule, which is bounded behind by the upturned end 

 (ramus marginalis) of the calloso-marginal sulcus and contains, near its hind border, 

 the termination of the fissure of Rolando. By means of an annectant convolution 

 passing below the last-named furrow, the frontal part of the paracentral lobule is con- 

 tinuous with the part contributed by the parietal lobe. The middle of the mar- 

 ginal gyrus is often incompletely subdivided by a shallow longitudinal groove, the 

 mesial frojital sulcus, into an upper and a lower tract, whilst its anterior and lower 

 end is uncertainly cleft by two or three short downward curving furrows, the siilci 

 rostrales. 



The orbital surface of the frontal lobe is marked by two fissures, the olfactory 

 and the orbital and by three chief convolutions, the inner, the middle and the outer 

 orbital. Although such division is convenient for the purposes of description, it 

 must be remembered that these orbital gyri are not separate convolutions, but largely 

 the inferior portions of the upper, middle and lower frontal convolutions of the outer 

 surface of the lobe. 



The olfactory sulcus lodges the olfactory bulb, tract and tubercle, and ex- 

 tends parallel with, or inclined somewhat towards the great longitudinal fissure. Its 

 course being straight, the sulcus marks off a narrow strip, about i cm. in width, 

 along the mesial border of the lobe. This area, although specially designated as the 

 gyrus rectus, is only a part of the broader longitudinal tract which corresponds to 

 the orbital surface of the superior frontal con\'olution. 



The orbital sulcus includes a number of furrows whose arrangement is very 

 variable, not only in different brains but often on the two sides of the same brain. 

 In the disposition assumed as the typical one, which, however, is far from constant, 

 the orbital sulcus consists of two longitudinal limbs, connected by a shorter trans- 

 verse arm, the three furrows forming a common fissure which corresponds more or 

 less closely with the letter H. In many cases, however, the sulcus more nearly re- 

 sembles an X or K, or it may be still further modified by the presence of additional 

 secondary grooves of variable number and length. Assuming the conventional H- 

 form to exist, the orbital surface is divided into three longitudinal tracts, the inner, 

 middle and outer orbital gyri, by the long limbs (sulcus orbitalis internus et exter- 

 nusj. The inner tract is subdivided by the olfactory sulcus into the gyrjis rectus , 

 above mentioned, and an outer part, the gyrus orbitalis i^iternus in the more 

 restricted sense. The middle orbital gyrus is subdivided by the curved transverse 

 limb r sulcus orbitalis transversus) into the anterior and the posterior orbital gyrus, 

 which lie respectively in front and behind the transverse furrow. In many cases the 

 latter curves outward and backward until it almost reaches the Sylvian fissure. 



