56 THE CEREBRUM. 



point, three-quarters of an inch below the tuber parietale, lies 

 directly over the posterior ramus of this fissure. 



The Sulcus Centralis (Rolandi, Figs. 22, 23, 24 and 27). 

 Beginning just above the posterior limb of the lateral cerebral 

 fissure, is the central sulcus, which extends upward and backward 

 to the longitudinal fissure of the cerebrum. Its upper extremity 

 is about half an inch (or 5.7 per cent.) behind the middle of a 

 line drawn from the nasal eminence to the external occipital 

 protuberance. With this sagittal meridian the sulcus centralis 

 forms an anterior angle of 69 to 74 degrees.' The average Rolandic 

 angle is 71 7' (Cunningham). The sulcus centralis is three 

 and three- eighths inches long and forms the boundary between 

 the frontal and the parietal lobe. It is developed hi two parts 

 a superior third and an inferior two-thirds, which join at an angle 

 open backward, called the genu superius ; both parts may present 

 an anterior concavity. Often a concealed gyrus separates the 

 two parts of the sulcus at the genu superius (Fig. 22). This 

 superior genu is in line with the superior frontal sulcus and marks 

 the probable location of the trunk center and the boundary between 

 the arm and leg areas in the anterior central gyrus. There is a 

 less constant angle, the genu inferius, hi the lower part of the 

 central sulcus; it is in line with the inferior frontal sulcus and 

 marks the lower limit of the arm area and the upper limit of the 

 face area. 



The Occipito-parietal Sulcus. (Sulcus occipito-parietalis). 

 If the line on the skull locating the posterior limb of the lateral 

 cerebral fissure be extended back to the sagittal meridian its 

 posterior end marks the location of the occipito-parietal sulcus. 

 The sulcus is located one- sixth of an inch above the lambda in 

 the adult, and is from one and a half to two inches above the 

 occipital pole. The greater part of the occipito-parietal sulcus 

 is situated on the medial surface of the cerebral hemisphere, hence, 

 it is divided into an internal part and an external part which are 

 continuous through the supero-medial border (Figs. 20, 22 and 28). 

 To the extent of its depth, which is about one inch, the external 

 occipito-parietal sulcus separates the occipital from the parietal 

 lobe on the convex surface of the hemisphere. Cunningham 



