THE CEREBRUM 403 



hemisphere, somewhat nearer the posterior than the anterior 

 end. It lies between two gyri which, though they are 

 obliquely placed, are still the most vertical gyri on the supero- 

 lateral surface, and it separates the frontal from the parietal 

 lobe. The upper end of the sulcus frequently cuts the 

 supero-medial border of the hemisphere, and, in such cases, 

 it appears on the medial surface of the hemisphere. The 

 lower end, as a rule, is separated from the posterior ramus of 

 the lateral fissure by a small bounding gyms. The sulcus 

 does not take a straight course between its two extremities ; 

 on the contrary, it is bent upon itself several times, on 

 account of buttress-like projections from its bounding walls 

 (Figs. 138, 153). The two most prominent buttresses spring 

 from the anterior wall, which is formed by the anterior central 

 gyrus. From its upper end the sulcus runs, at first, downwards 

 and forwards to the base of the upper buttress ; then it bends, 

 first round the upper and next round the lower buttress; 

 finally, its lowest part runs almost vertically downwards from 

 the base of the lower buttress. The anterior central gyrus, 

 which forms the anterior boundary of the central sulcus, 

 constitutes the motor region of the cerebral cortex, and by 

 means of the buttresses which spring from its posterior face 

 it is possible to define in a fairly accurate manner the various 

 motor areas. The lower limb area extends from the upper 

 end of the central sulcus to the apex of the upper buttress ; 

 the trunk area corresponds with the lower face of the upper 

 buttress ; the upper limb area corresponds with the region of 

 the lower buttress ; and the head area corresponds with the 

 anterior boundary of the central sulcus below the lower 

 buttress. 



When the margins of the central sulcus are gently separated, 

 a transverse annectant gyrus will be found crossing its floor 

 and uniting together the anterior and posterior central gyri. 

 It lies at the level of the lower part of the upper buttress of 

 the anterior central gyrus. 



Fissura Parieto-occipitalis. The greater part of the 

 parieto-occipital sulcus is situated on the medial surface of 

 the cerebral hemisphere (Fig. 159); only the small lateral 

 part appears on the supero-lateral face (Fig. 153). 



The lateral part. of the parielo-occipital fissure (O.T. external 

 parieto-occipital] cuts the supero-medial border of the hemi- 

 sphere, in a transverse direction, from 37.5 to 50 mm. (one 

 in 26 a 



