PARIETAL LOBE. ]49 



fissure, and is partially interrupted by the ascending branches of that fissure. Besides 

 these, there is often, but not constantly, a sagittal fissure running along the middle of 

 the convolution. This fissure (sukus frontalis mesialis s. marginalia) subdivides the 

 middle of the gyrus into superior and inferior parts. The antero-inferior part of 

 the marginal gyrus is marked by two or three sulci which are curved concentrically 

 with the prelimbic part of the calloso-marginal sulcus. These have been termed by 

 Eberstaller the rostral sulci (supra-orbital of Broca) (fig. 102, ro., ro. inf.) Below 

 the genu of the callosum the marginal gyrus is continuous with the limbic lobe 

 by one or more broad fronto-limbic annectent gyri. Other annectents are found 

 between the several parts of the calloso-marginal sulcus, when this is subdivided ; 

 when it is not subdivided they are concealed in its depth. At its posterior end the 

 marginal gyrus becomes continuous with the two central gyri on either side of the 

 upper extremity of the fissure of Rolando. But this posterior part of the marginal 

 gyrus is nearly cut off from the rest of the gyrus by the anterior ascending part of the 

 paracentral sulcus, and is distinguished as the paracentral or oval lobule (fig. 102). 



SULCI AND GYRI OF THE ORBITAL SURFACE. The orbital sulcus (H-shaped 

 sulcus, triradiate sulcus) is a complex of sulci, which extends over the greater part 

 of the flattened orbital surface of the frontal lobe. It consists of a posterior part 

 (sulcus orbitalis transversus of Weisbach, fig. 103, o.tr), which curves round from near 

 the anterior Sylvian fissure laterally, in an arch convex forwards, to end mesially 

 near the lateral root of the olfactory tract ; and of three or four sulci having a 

 general sagittal direction (sulci orbitales sagittales), and usually communicating 

 posteriorly with the transverse orbital. These sagittal sulci vary greatly in their 

 number and extent, and in the number and direction of the secondary and tertiary 

 sulci, which come off from them, and they frequently produce, with the transverse 

 sulcus, the figure of an H, K or X. Weisbach has endeavoured to connect these 

 variations with racial and sexual differences, but there does not seem to be any 

 foundation for such connexion, and in fact (as is well shown in fig. 103), two 

 entirely different types may be found on opposite hemispheres of the same brain. 



Three convolutions are described as lying in front of the transverse orbital 

 sulcus, named, according to their position, the inner, middle, and outer orbital 

 gyri. The last of these is continuous at the side with the inferior frontal, the first 

 and second are the continuations of the superior and middle frontal convolutions. 



On the inner gyrus is seen the olfactory sulcus (s.olf.) in which the olfactory 

 tract and bulb lie. It has a straight course nearly parallel with the great longitudinal 

 fissure, but somewhat inclining towards it. The part of the inner gyrus between 

 the olfactory sulcus and the mesial border is sometimes known as the gyrus rectus (R). 

 Behind the transverse orbital sulcus, between it and the anterior limiting sulcus of 

 the insula, is a fourth gyrus, the posterior orbital, which is also continuous with 

 the inferior frontal. 



The continuity of the convolutions of the external with those of the orbital surface is often 

 interrupted by a sulcus which crosses their direction, extending across the front of the lobe 

 from the anterior end of the Sylvian (sulcus fronto-orbitalis, Griacomini ; fronto-marglnalls 

 lateralis, Eberstaller). 



PAEIETAL LOBE. This lobe lies behind the frontal, in front of the occipital, 

 and above the temporal lobe. It is bounded in front by the fissure of Rolando, 

 behind by the parieto-occipital and anterior occipital fissures (p. 152), laterally and 

 below by the posterior limb of the fissure of Sylvius as far as this preserves its 

 horizontal direction, and then by a line connecting this with the lower end of the 

 anterior occipital, but it is here freely continuous with the temporal lobe.. Above 

 and mesially it extends within the great longitudinal fissure and appears on the 

 mesial side of the hemisphere, as the quadrate lobule. 



SULCI AND GYRI OF THE PARIETAL LOBE. The intraparietal sulcus 



