822 



NEUROLOGY 



posterior sulcus, the paramedial sulcus, which, however, is frequently interrupted 

 by bridging gyri. 



The middle frontal gyrus (gyms frontalis medius; medifrontal gyre), between the 

 superior and inferior frontal sulci, is continuous with the anterior orbital gyrus on 

 the inferior surface of the hemisphere; it is frequently subdivided into two by a 

 horizontal sulcus, the medial frontal sulcus of Eberstaller, which ends anteriorly in 

 a wide bifurcation. 



The inferior frontal gyrus (gyrus frontalis inferior; subfrontal gyre) lies below the 

 inferior frontal sulcus, and extends forward from the lower part of the precentral 

 sulcus; it is continuous with the lateral and posterior orbital gyri on the under 

 surface of the lobe. It is subdivided by the anterior horizontal and ascending rami 

 of the lateral fissure into three parts, viz., (1) the orbital part, below the anterior 

 horizontal ramus of the fissure; (2) the triangular part (cap of Broca), between 

 the ascending and horizontal rami; and (3) the basilar part, behind the anterior 

 ascending ramus. The left inferior frontal gyrus is, as a rule, more highly 

 developed than the right, and is named the gyrus of Broca, from the fact that 

 Broca described it as the center for articulate speech. 



The inferior or orbital surface of the frontal lobe is concave, and rests on the orbital 

 plate of the frontal bone (Fig. 729) . It is divided into four orbital gyri by a well- 

 marked H-shaped orbital sulcus. These are 

 named, from their position, the medial, anterior, 

 lateral, and posterior orbital gyri. The medial 

 orbital gyrus presents a well-marked antero- 

 posterior suleus, the olfactory sulcus, for the 

 olfactory tract; the portion medial to this is 

 named the straight gyrus, and is continuous with 

 the superior frontal gyrus on the medial surface. 

 The medial surface of the frontal lobe is occu- 

 pied by the medial part of the superior frontal 

 gyrus (marginal gyrus) (Fig. 727). It lies be- 

 tween the cingulate sulcus and the supero-medial 

 margin of the hemisphere. The posterior part 

 of this gyrus is sometimes marked off by a ver- 

 tical sulcus, and is distinguished as the paracen- 

 tral lobule, because it is continuous with the 

 anterior and posterior central gyri. 



Parietal Lobe (lobus parietalis) . The parietal 

 lobe is separated from the frontal lobe by the 

 central sulcus, but its boundaries below and 



behind are not so definite. Posteriorly, it is limited by the parietooccipital fissure, 

 and by a line carried across the hemisphere from the end of this fissure toward 

 the preoccipital notch. Below, it is separated from the .temporal lobe by the 

 posterior ramus of the lateral fissure, and by a line carried backward from it to 

 meet the line passing downward to the preoccipital notch. 



The lateral surface of the parietal lobe (Fig. 726) is cleft by a well-marked furrow, 

 the intraparietal sulcus of Turner, which consists of an oblique and a horizontal 

 portion. The oblique part is named the postcentral sulcus, and commences below, 

 about midway between the lower end of the central sulcus and the upturned end 

 of the lateral fissure. It runs upward and backward, parallel to the central sulcus, 

 and is sometimes divided into an upper and a lower ramus. It forms the hinder 

 limit of the posterior central gyrus. 



From about the middle of the postcentral sulcus, or from the upper end of its 

 inferior ramus, the horizontal portion of the intraparietal sulcus is carried backward 

 and slightly upward on the parietal lobe, and is prolonged, under the name of the 





FIG. 729. Orbital surface of left frontal lobe. 



