664 



THE NERVOUS SYSTEM. 



doubled lines which are less dense than those of the sensory cortex ; this area forms 

 the crest of the gyms centralis posterior, and then gives place to another slightly 

 modified type of cortex which forms the anterior wall of the sulcus postcentralis. 

 Thus the sensory cortex has two fringing bands analogous to those already noticed 

 alongside the visual and acoustic areas. 



The motor and sensory areas cross on to the medial aspect of the hemisphere, 



into a region known as the lobulus paracentralis. 



Sulcus centralis T j_i p i 



Gyrus centralis anterior | Gyrus centralis posterior In thlS region a lUrrOW (SUlcUS paracentrallS) 



is sometimes found along the line of demarca- 

 tion of the medial parts of the motor and 

 sensory areas (Fig. 589). 



That portion of the parietal region which 

 intervenes between the gyrus centralis pos- 

 terior and the occipital region is usually 

 subdivided into two distinct parts (lobulus 

 parietalis superior and lobulus parietalis in- 

 ferior) by a horizontal furrow, called the 

 sulcus interparietalis proprius. The term 

 sulcus interparietalis is usually applied in a 

 purely arbitrary and artificial manner to a 

 complex of four genetically distinct and in- 

 dependent furrows (Fig. 593, p 1 , p 2 , p*, and p*\ 

 the Sulcus Postcentralis inferior (^ ), the sulcus 

 postcentralis superior (p*), the sulcus inter- 

 parietalis proprius (ramus horizontalis) (p 3 \ 



and the sulcus paroccipitalis (ramus occipitalis) (p*\ which ends in the sulcus 

 occipitalis transversus. 



These four furrows develop quite independently one of the other, the postcentral 

 sulci as the posterior boundary of the sensory territory, the paroccipital sulcus as 

 the supero-lateral boundary of the visual territory, and the more variable horizontal 

 ramus (the sulcus interparietalis, in the strict sense of the term) as a demarcation 



Transitional gyrus Par ie to-occipital sulcus (incisura) 



Posterior central gyru 



Transitional / 

 gyri I 



Boundary line between motor and sensory cortex 



FIG. 592. SECTION ACROSS THE SUPERIOR PART 

 OF THE SULCUS CENTRALIS IN A FRESH BRAIN. 



Termination of lateral fissure 



FIG. 593. THE FOUR SULCI OF THE INTERPARIETAL COMPLEX OPENED UP, so as to show the deep 

 transitional gyri intervening between them. 



p 1 . Sulcus' postcentralis inferior. 

 p 2 . Sulcus' postcentralis superior. 



p 3 . Ramus horizontalis (sulcus interparietalis proprius). 

 p 4 . Ramus occipitalis (sulcus paroccipitalis). 



between the two parietal lobules. The four furrows may unite to form any possible 

 combination. 



The superior parietal lobule is composed of moderately thick cortex (2'5 to 

 3 mm.) placed between the interparietal sulcus (ramus horizontalis) and the superior 

 border of the hemisphere,, where it becomes continuous on the medial surface with 

 the precuneus. Each of these parts is' subdivided by a transverse sulcus, the 



