EXTEKXAL SURFACE. 527 



The superior, ov first frontal convolution (s. fr. c), at the margin of the 

 great longitudinal fissnre, commonly commences superficially at the upper 

 end of the ascending frontal convolution, and extends to the anterior 

 extremity of the hemisphere, where, tapering, it passes over to the 

 oi'bital surface. The inner aspect of this convolution appears on the 

 median surface of the hemisphere. 



The miihlle or second frontal convolution (m. fr. c.) arises deeply or super- 

 ficially fi'om the ascending frontal below the last, and runs thence to the 

 anterior extremity of the hemisphere. It is usually broad, and often 

 much subdivided. 



The inferior or third fronted convolution (i. fr. c.) forms the lower and 

 outer portion of the frontal lobe. It arises superficially, or sometimes 

 deeply, fi'om the lower extremity of the ascending convolution, just 

 above the bifurcation of the fissure of Sylvius, arches round the ascend- 

 ing limb of that fissure, and extends to the anterior extremity of the 

 hemisphere. 



Fissures of the frontal surface. — The transverse frontal fissure, " pra3- 

 central sulcus," lies in front of the ascending fi'ontal convolution, and 

 parallel to the fissure of Rolando. Its extent depends on the mode of 

 origin of the superior, middle, and inferior frontal convolutions from 

 the ascending frontal. When these arise superficially the fissure is 

 interrupted, and may be inconspicuous ; when the inferior frontal con- 

 volution arises deeply, this fissure is continuous with the fissure of 

 Sylvius, of which it has, in consequence, been regarded as the ascending 

 limb. 



Two antero-posterior fissures, the superior and inferior fronted sepa- 

 rate the corresponding convolutions from the middle frontal: they are 

 often very irregular, being bridged over by secondary convolutions. 



OnBiTAL S^'KF ACE {fig. olb) 'pvQ?,e\\i% iv;o fissures ; the olfactonj sulcus, 

 straight, parallel with the great longitudinal fissure, and lodging the 

 olfactory bulbs. 



The orbital sulcus, irregular, often triradiate, lying in the centre of 

 the lobule. 



Convolutions. — Between the olfactory sulcus and the longitudinal 

 fissure is the sfraif/ht convolution, continuous, at its anterior extremity, 

 with the superior frontal. 



Three convolutions are sometimes described as lying around the 

 orbital sulcus, and named according to their position, the inner, the 

 anterior, and the outer or posterior orbital convolutions. 



The Parietal Lobe (fig. 370) lies behind the frontal and in front 

 of the occipital lobe. Below it is the temporo-sphenoidal lobe. It is 

 bounded in front by the fissure of Rolando, behind by the parieto- 

 occipital fissure, and by an arbitrary continuation of the line of that 

 fissure to the outer boundary. Internally it is bounded by the great 

 longitudinal fissure, and externally by the posterior limb of the fissure 

 of Sylvius as far as this preserves its horizontal direction, and then 

 by a line continuing that direction to the posterior boundary. 



Fissure. The intra-parietal fissure (fig. 370, int. par. f.) arches through 

 the parietal lobe, commencing in its anterior inferior angle, where it is 

 sometimes, though rarely, continuous with the fissure of Sylvius. It 

 ascends at first parallel to the fissure of Rolando, and then turns back- 

 wards horizontally to the back of the lobe, extending nearly to the termi- 

 nation of the external portion of the parie to-occipital fissure, past which 



