THE FRONTAL LOBE 



857 



anterior perforated substance and the vallecula Sylvii, and is known as the 

 limen of the insula. In the folding process by which the opercula accomplish 

 the overlapping and enclosing of the island, there results a deep sulcus which sur- 

 rounds its entire area except at the limen insulse. This is known as the circular 

 sulcus, or limiting sulcus of Reil. - The gyri (and sulci) of the insula radiate from 

 the apex of the triangle. The central sulcus of the insula is the deepest. It runs 

 from below backward and upward, parallel with the central sulcus of Rolando 

 above and divides the insula into a larger anterior and a smaller posterior portion. 

 The anterior portion consists of from three to five short irregular gyri breves or 

 precentral gyri, separated by sulci brevis ; the posterior portion consists of a single, 

 slightly furrowed gyrus, which is long and arched and extends from the apex to 

 the base of the triangle, the gyrus longus. 



In a recent study of the insula of more than 200 human brains, including a few of idiots 

 and paralytics and a series of young foetuses, Xelidoff finds that the left island is more deeply 

 marked bj^ sulci and averages 11 mm. longer than the right; that, of the sulci in the island, the 

 central sulcus is the first to appear, is the most persistent sulcus in defective brains, though 

 occasionally absent in microcephalic idiots, and that in the average it is more pronounced 

 in males than in females. 



The frontal lobe.— This is the most anterior of the lobes of the hemisphere, 

 and like the two lobes behind, it has a convex or lateral, a basal, and a mesial 

 surface. The convex surface begins with the frontal pole, and is bounded 

 posteriorly by the central sulcus {Rolandi). The basal surface extends backward 

 to the stem of the lateral fissure, covered by the frontal pole. The mesial surface 

 is separated from the gyrus cinguli of the rhinencephalon (limbic lobe) by the sub- 

 frontal part of the sulcus cinguli (calloso-marginal fissure), and from the parietal 

 lobe by a hne drawn perpendicularly from the upper extremity of the central 

 sulcus (Rolandi) to the sulcus cinguli. These surfaces include the following gyv\ 

 and sulci: — 



Convex 

 surface 



Gyri. 



Anterior central gyrus. 

 Superior frontal gyrus. 



Middle frontal gyrus < 

 Inferior frontal gyrus \ 



Sulci. 



Precentral sulcus 



Superior portion. 

 Inferior portion. 

 Opercular portion. 

 Triangular portion. 

 Orbital portion. 



Basal 

 surface 



Mesial 

 surface 



Orbital gyri 



{ Lateral. 



i Anterior. 



I Posterior. 



i IMedial. 

 GjTus rectus. 

 Superior frontal gjTus. 

 Marginal gyrus. 

 Paracentral lobule (anterior part). 



Superior. 

 Inferior. 

 Superior frontal sulcus. 

 Middle frontal sulcus. 

 Inferior frontal sulcus. 

 Anterior ascending ramus of lateral 



fissure. 

 Anterior horizontal ramus of lat- 

 eral fissure. 



Lateral. 

 Orbital sulci \ Medial. 



Transverse. 



Olfactory sulcus. 



Rostral sulci. 



Many of the sulci, especially the superior frontal and the rostral sulci, often 

 give off twigs or are broken up into short furrows which give rise to small folds 

 [gyri transitivi], too inconstant to be given special names. 



The anterior central gyrus (ascending frontal convolution) is the only gyrus 

 of the frontal lobe having a vertical direction. It lies parallel to the central sulcus 

 (Rolandi), and thus extends obliquely across the convex surface from the posterior 

 ramus of the lateral fissure (frontal operculum) to the supero-mesial border, and is 

 continuous on the mesial surface with the anterior portion of the yara-central 

 lobule. It comprises the larger part of the motor portion of the somsesthetic 

 (sensory-motor) area of the cerebral cortex. It is separated from the horizontal 

 frontal gyri in front of it by the precentral sulcus. 



This sulcus is developed in three parts, but the upper and middle parts in the foetal brain 

 usually fuse together, so that in the later condition it consists of a superior and an inferior 

 segment. The superior cuts the supero-mesial border of the hemisphere and appears on the 

 mesial surface in the paracentral lobule. On the convex surface it is usually connected with the 

 posterior end of the superior frontal sulcus (fig. 674). 



The superior frontal gyrus is a relatively broad, uneven convolution, com- 

 prising the anterior portion of the supero-mesial border of the hemisphere, and 

 therefore extends horizontally from the precentral sulcus to the frontal pole. It 

 is sometimes inperfectly divided into a superior and an inferior part by a series of 



