CRANIAL CAVITY. 



face : In front of the fissure of Rolando is the anterior central 

 convolution, which is vertical and placed behind the precentral 

 sulci ; of the latter the superior is short and vertical, while the 

 inferior has a vertical and a horizontal part. Anteriorly two 

 horizontal sulci, the superior and inferior frontal, mark off 

 three gyri, the superior, middle, and inferior frontal ; the sulcus 

 paramedialis and sulcus frontalis medius divide the superior 

 and middle gyri respectively into upper and lower parts; the 

 anterior Sylvian limbs divide the inferior gyrus, from behind 

 forward, into the pars basilaris, pars triangularis and pars 

 orbitalis. [560] 



Mesial surface : This comprises the gyrus marginalis, which 

 lies between the supero-mesial border and calloso-marginal fis- 

 sure; one or two curved sulci usually appear anteriorly; the 

 posterior part (paracentral lobule) is partly cut off and pre- 

 sents the end of the fissure of Rolando. Orbital surface: The 

 gyrus rectus lies between the mesial border and olfactory sul- 

 cus (for the olfactory tract and bulb) . Externally three orbital 

 gyri (internal, anterior, and posterior) are marked off by the 

 orbital sulcus; this is H-shaped and formed by three limbs, 

 internal, transverse, and external. [561] 



Parietal Lobe. This is bounded on the outer surface: in 

 front by the fissure of Rolando; below by the posterior Syl- 

 vian limb and an imaginary prolongation of it backward; and 

 behind by the external parieto-occipital fissure and an imag- 

 inary prolongation of it downward to an identation (prseoccip- 

 ital notch) on the infero-lateral border. The mesial surface 

 is the quadrate lobule (praecuneus) ; this lies between the 

 parieto-occipital fissure and upturned end of the calloso-mar- 

 ginal fissure, and is bounded below by a variable postlimbic 

 sulcus. [562] 



External Surface. This presents a T-shaped sulcus (intra- 

 parietal) formed by four, usually connected, sulci as follows: 

 vertically in front are the superior and inferior postcentral sulci; 

 and running backward from between these is the ramus hori- 



[39] 



