858 THE NERVOUS SYSTEM 



detached, irregular furrows, spoken of collectively as the para-medial sulcus. 

 The resulting transitory gyri are of considerable 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 temporal pole. It is separated from the superior 

 frontal gyrus bj^ the superior frontal sulcus, which is usually continuous into the 

 superior section of the precentral sulcus and thence extends horizontally to the 

 frontal pole. The middle frontal gyrus is in most cases subdivided anteriorly 

 into a superior and an inferior -portion by a middle frontal sulcus. This sulcus begins 

 above and runs into the frontal pole, where, upon reaching the superciliary border, 

 it frequently bifurcates into a transverse furrow, known as the fronto-marginal 

 sulcus. 



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

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

 sulcus usually begins continuous with the inferior section of the precentral sulcus, 

 and extends, very irregularly and frequently interrupted, toward 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 opercu- 

 lum 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 be- 

 tween the two anterior rami of the lateral fissure is the cap-shaped triangular 

 portion. This portion frequently involves 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, the motor area 

 for the function of speech. The area controlling speech, however, involves that part of the 

 triangular portion bounding the anterior ascending ramus of the lateral fissure 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 no doubt influences the development of the frontal opercula. 



On the basal surface (fig. 676) 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 comprise a medial, a lateral and a transverse orbtial sulcus. 

 This figure naturally divides the orbital area into four gyri: — (1) The lateral 

 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 insulse and the stem of the lateral 

 fissure, and its outer part is in relation with the orbital portion of the operculum; 

 (4) the medial 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 pajt of the parol- 

 factory area or Broca's area, which functionally belongs to the rhinencephalon. 

 As an area or field, this appears mesially lying between the anterior and posterior 

 pai(jlfactory sulci. 



On the mesial surface (fig. 679), of the frontal lobe the superior frontal gyrus is 

 separatefl frf)m the gyrus cinguli of the rhinencephalon (limbic lobe) by the well- 

 marked sulcus cinguli. Aiitc^riorly the superior frontal gyrus is su])divide(l by the 

 main stem of the rostral sulci into a marginal gyrus, and what may be termed a 

 submarginal gyrus. Tlie marginal gyrus is usually broken into smaller parts by 

 twigs of th(! rostral sulci, most of which arc perpendicular to the main stem, while 

 the submarginal gyrus is less frequently interrupted. Posteriorly the superior 

 frontal gyrus constitutes the anterior portion of the paracentral lobule, a part of 



