838 THE NERVOUS SYSTEM 



into the frontal pole, where, upon reaching the superciliary border, it frequently 

 bifurcates into a transverse furrow, known as the fronto-marginal sulctts. 



The inferior frontal gyms forms the superior wall of the lateral fissure, and is 

 separated from the middle frontal gyrus by the inferior frontal sulcus. This sulcus 

 begins continuous with the inferior section of the precentral sulcus, and extends, 

 very irregularly and frequently interrupted, towards the frontal pole. The gyrus 

 abuts upon the anterior central gyrus, and its posterior portion is divided into three 

 parts (the frontal opercula) by the anterior ascending and horizontal rami of the 

 lateral fissure. The part behind the anterior ascending ramus is the opercular 

 portion (a part of the fronto-parietal operculum or operculum proper), sometimes 

 referred to as the basilar portion. In most brains this part is traversed by a short 

 oblique furrow, the diagonal sulcus. The part between the two anterior rami of the 

 lateral fissure is the cap-shaped triangular portion. This portion frequently in- 

 volves one and sometimes two descending twigs of the inferior frontal sulcus. The 

 part below the anterior horizontal ramus is by position the orbital portion. 



It is seen that the inferior frontal gyrus gives rise to the whole of the frontal operculum 

 and the anterior half of the fronto-parietal operculum. The opercular portion is of special 

 interest in that in the left hemisphere it constitutes the celebrated convolution of Broca, con- 

 cerned in the function of speech. The area controlling speech, however, involves the triangular 

 portion as well, and both these parts often appear more developed on the left hemisphere. The 

 development of the opercula of the inferior frontal gyrus is a distinctive characteristic of the 

 human brain. This gyrus does not develop opercula even in the highest varieties of apes. The 

 development of the function of speech in man rio doubt influences the development of the frontal 

 opercula. 



On the basal surface of the frontal lobe is the orbital area and the gyrus rectus. 

 The more pronounced of the orbital sulci are often so joined with each other as to 

 form an H -shaped figure standing parallel to the mesial plane, and thus they com- 

 prise an internal, an external, and a transverse orbital sulcus. This figure naturally 

 divides the orbital area into four gyri: (1) The external orbital gyrus is the basal 

 continuation of the inferior frontal gyrus, and is thus related to the orbital portion of 

 the frontal operculum; (2) the anterior orbital gyrus is continuous at the pole with 

 the middle frontal gyrus; (3) the posterior orbital gyrus is closely related to the 

 limen insulffi and the stem of the lateral fissure, and its outer part is in relation with 

 the orbital portion of the operculum; (4) the internal orbital gyrus is continuous 

 over the superciliary border with the superior frontal gyrus. It frequently contains 

 one or two short, isolated sulci. Its mesial boundary is the straight olfactory sulcus, 

 in which lies the olfactory bulb and tract of the rhinencephalon. This sulcus marks 

 off a narrow straight strip of cortex between it and the mesial border of the lobe 

 known as the gyrus rectus. The posterior portion of the gyrus rectus comprises a 

 part of the parolfactory area or Broca's area, which functionally belongs to the 

 rhinencephalon. 



On the mesial surface of the frontal lobe the superior frontal gyrus is separated 

 from the gyrus cinguli of the rhinencephalon (limbic lobe) by the well-marked sulcus 

 cinguli. Anteriorly the superior frontal gyrus is subdivided by the main stem of 

 the rostral sulci into a marginal gyrus, and what may be termed a submarginal 

 gyrus. The marginal gyrus is usually broken into smaller parts by twigs of the 

 rostral sulci, most of which are perpendicular to the main stem, while the submar- 

 ginal gyrus is less frequently interrupted. Posteriorly the superior frontal gyrus 

 constitutes the anterior portion of the paracentral lobule, a part of the somresthetic 

 area of the mesial surface of the hemisphere. This lobule is usually marked off an- 

 teriorly by a vertical twig from the sulcus cinguli. 



The sulcus cinguli (calloso-marginal fissure) is the longest and one of the most 

 prominent sulci on the mesial surface of the hemisphere. It divides the anterior 

 portion of the mesial surface into a marginal part above and a callosal part below 

 in other words, it separates the superior frontal gyrus from the gyrus cinguli. Its 

 subfrontal portion begins below the rostrum of the corpus callosum and curves 

 forwards and upwards around the genu, and then turns backwards above the body of 

 the corpus callosum. Before it reaches the level of the splenium, it turns upwards 

 and cuts and terminates in the supero-mesial border of the hemisphere, as the next 

 sulcus behind the upper termination of the central sulcus. This upward turn is the 

 marginal portion of the sulcus cinguli. It is sometimes an abrupt curve and some- 

 times curves gradually, but its marginal relation to the upper end of the central sulcus 



