Till'. ( /;/.' /:/;/,'. I /. HEMISPHERES 



S.-57 



parallel with the central sulcus of Kolamlo above, and divides the island 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 breves; 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. 



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 outer, a basal, and a mesial 

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

 by the central sidcus (liolaiuli) ; the basal surface extends backwards to the stem of 

 the lateral fissure, covered by the frontal pole; and the mesial surface is separated 

 from the gyms 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 line 

 drawn perpendicularly from the upper extremity of the central suleus (Rolandi) to 

 the sulcus cinguli. These surfaces include the following gyri and sulci: 



Convex 



surface 



Bun) 



surface 



Mesial 

 surface 



GYRI. 



Anterior central gyrus. 

 Superior frontal gyrus. 



Middle frontal gyms ) ^"I^rior v~. 



J I Inferior ]Mirtion. 



!l >percular ]Kirtion. 

 Triangular jxlrtiou. 

 Orbital jxirtion. 



! Orbital gyri 



[ Gyrns rectus. 



Superior frontal gyrns. 



External. 

 Anterior. 

 Posterior. 

 Internal. 



Marginal gyrns. 



I'araccntnil lobule (anterior part). 



SlI.CI. 



BBS- 



Superior frontal sulcus. 



Middle frontal Mileii- 



Inferior frontal sulcus. 



Anteiior ascending ranius of lateral 



li-sure. 

 Anterioi- boi i/.ontal ranius of lateral 



tissure. 



| IMenial. 

 Orbital sulci < Internal. 



(. Transverse. 



Olfactory sulcua. 



Kostral 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 

 ranius of the lateral fissure (frontal o|H>rculum) to the supero-mesial border, and is 

 continuous on the mesial surface into the anterior portion of the para-central lobut> . 

 It comprises a large part of the anterior portion of the soma'sthetic (sensory-motor) 

 area of the cerebral cortex. It is separated from the horizontal gyri in front by the 

 precentral sulcus. This sulcus is developed in three parts, but the upper and 

 middle parts in the I'u'tal brain usually fuse together, so that in the later condition 

 it consists. of a superior and an inferior section. The superior cuts the supero-mesial 

 border of I he 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. 



The superior frontal gyrus is a relat ively broad, uneven convolution, comprising 

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

 extends hori/.ontally from the precentral sulcus to the frontal pole.- It is sometimes 

 imperfectly divided into a superior and an inferior part by a series of detached, ir- 

 regular furrows, spoken of collect ively as the /xmi-nni/iul xulctis. The resulting gyri 

 inmsitivi are of interest in that they are peculiar to the human brain, and are said to 

 be more marked in the higher than in the lower types. 



The middle frontal gyrus is likewise a broad strip of pallium extending from the 

 precentral sulcus to the frontal pole. It is separated from the superior frontal 

 gyrns by the superior frontal sulcus, which is usually continuous into the superior 

 section of the precentral sulcus and thence extend.- hori/ontally to the frontal pole. 

 The middle frontal gyrus is in most cases subdivided anteriorly into a superior and an 



rior portion by a middle frontal sulcus. This sulcus begins above .and runs 



