860 THE NERVOUS SYSTEM 



upper end terminates in and usually cuts the supero-mesial border of the hemis- 

 phere immediate^ in front of the termination of the marginal portion of the sulcus 

 cinguli. Thence it pursues an oblique though sinuous course forward across the 

 convex surface of the hemisphere, forming on the average an angle of about 72° 

 with the supero-mesial border (Rolandic angle), and terminates in the fronto- 

 parietal operculum immediately above the posterior ramus, and about 2.5 cm. be- 

 hind the point of origin of the anterior rami of the lateral fissure. It rarely cuts 

 through the fronto-parietal operculum. In its sinuous course, varying from the 

 line of its supero-mesial end, two bends are marked (fig. 677) : — (1) The superior 

 genu occurs at about the junction of the upper and middle thirds of the sulcus and 

 is concave forward. It accommodates the greater part of that portion of the cor- 

 tex which is the motor area for the muscles of the leg and trunk, and the develop- 

 ment of this area in man probably aids in producing it. (2) The inferior genu 

 occurs below, is concave forward and is commonly a little more marked than the 

 superior genu. It is probably in part a result of the superior genu — the turn of the 

 sulcus in resuming its general course, but it may further result from the develop- 

 ment of the shoulder and arm area of the cortex which occupies its concavity. 



Fig. 677. — Diagram Representing the Most Common Form of the Central Sulcus and 

 Indicating the Regions of Junction upon it of the Areas of the Precentral Gyrus 

 Devoted to the Different Regions op the Body, as Estimated by Symington and 

 Crymble. 



Superior-mesial border of hemisphere ' ~1^' Jk^ 



Regionof junction of leg and trunk areas ~__ I Superior genu 



Region of junction of trunk and .^r 



arm areas # 



Region of junction of arm and ^^ " Inferior genu 



face areas — - ,^^ 



Lateral end of sulcus 



Operculum ' 



The central sulcus (Rolandi) appears in the pallium of the foetus during the latter part of 

 the fifth month. It then consists of a lower longer and an upper shorter part. Usually these 

 two parts become continuous before birth; very rarely do they remain separate in the adult. 

 The point of their fusion is sometimes manifest within the depth of the sulcus. If the lips of 

 the sulcus be pressed widely apart at about the region of the junction of its middle and upper 

 thirds, it will be found that the opposing walls give off a number of protuberances or lateral gyri, 

 which dovetail into each other when the sulcus is closed. Sometimes two of these lateral gyri 

 appear fused across the floor of the sulcus, so as to form a bridge of grey substance known as the 

 deep annectant gyrus. This interruption of the continuity of the sulcus, when present, repre- 

 sents the point at which the two parts of tlic sulcus in the fcrtal brain joined each other without 

 the continuity becoming wholly completed in the adult. The genua of the adult sulcus maj"" 

 often be due to the precedent parts not being in line at the time of their fusion. 



P>om a special study of the central sulcus of 237 normal adult hemispheres, Symington 

 and Crymble (1913) give the following details: (1) that the most common course of the sulcus 

 is that illustrated in fig. G77, above; (2) that it varies in depth both in a given specimen and in 

 different specimens — the greatest variations in depth reported for a given sulcus being from 

 22 to 12 millimeters, the shallowest part being in the region of the deep annectant gyrus; (3) 

 that the average lengtii fi-om the supero-mesial border of the hemisphere to the opercular 

 end of the sulcus is al)out 9 cm. in direct line and 10.4 cm. following the curves of the sulcus. 

 The average lengtii of the curved door is 7.9 cm. (4) From the supro-meisal end of the sulcus 

 to the points of junction of th(> general areas of the precentral gyrus, direct line measurements 

 give averages, (a) to the junction of leg and trunk areas, 3. .5 cm.; (b) to junction of trunk and 

 arm areas, 4,5 cm.; (cj to junction of arm and face areas, 7.5 cm. 



The parietal lobe. — The parietal lo])e occupies a somewhat smaller area of 

 the; liuiiian t('lcM(;ci)halon than either the frontal or the temporal lobe. It has a 

 convex and a mesial surface, but no basal surface. It is separated from the 

 frontal lobe in front by the central sulcus; from the occipital lobe behind, on the 

 mesial surface by the parieto-occipital fissure (fig. 650), and, on the convex surface, 



