660 



THE NEKVOUS SYSTEM. 



limitans ; whereas the stria extends throughout both walls of the posterior calcarine 

 sulcus, and in most cases beyond its lips on to the surface of the cuneus and the 

 gyms lingualis (Figs. 588 and 589), i.e. the exposed cortical areas placed upon the 

 superior and inferior aspects respectively of the sulcus calcarinus posterior. 



Along the superior and inferior boundary lines of this area shallow limiting 

 sulci usually develop (Fig. 589), and these furrows often pass backwards into 

 little arched sulci polar es, which are furrows of the operculated variety (see 

 p. 646), called into existence by the broadening out of the area striata (not 

 an actual broadening, but an unfolding) as it passes round the edge of the 

 hemisphere. 



At the point of transition from the deep sulcus calcarinus into the shallower 

 sulcus calcarinus posterior (Fig. 588) a submerged ridge is usually found the gyrus 

 cuneolingualis anterior; and other similar ridges, which may be exposed on the 



Praecuneus 



Parieto-occipital fossa 



Inctf urai paneio-occipitalis 



Cunei 



Anterior cuneo- Jp; 



lingual gyrus 

 Sulcus calcarinus fc| 



posterior 



Posterior cuneo- 3^ 

 lingual gyrus 



I Gyrus cinguli 



j 



Corpus callosum 



Thalamus 

 Gyrus cunei 



Sulcus calcarinus 



Gyrus lingualis 



Sulcus lingualis 

 inferior 



Fia. 588. THE PARIETO-OCCIPITAL AND THE CALCARINE SULCI FULLY OPENED UP, so as to show the 

 deep transitional gyri marking off the several elements of the -<-shaped system. 



Area striata, uniform blue ; area parastriata, large blue spots ; area peristriata, fine blue dots. 



surface or may be submerged, are often found interrupting the posterior and lateral 

 calcarine sulci themselves (Fig. 588). 



The posterior and lateral calcarine sulci are subject to a very wide range of 

 variation in form, but they are always axial foldings of the area striata. 



When the area striata crosses on to the lateral surface of the hemisphere a small 

 semilunar furrow develops a short distance in front of its anterior edge. This is 

 the sulcus lunatus. The larger the lateral extension the closer does the edge of the 

 area striata approximate to the caudal lip of the sulcus, which under such circum- 

 stances assumes a definitely operculated form. Such cases occur most often in the 

 left hemisphere and in the brains of primitive people ; and they represent a perfect 

 realisation of a furrow once supposed not to occur in the human brain, but to be 

 distinctive of the ape. Hence it used to be called the " Affenspalte " or sulcus 

 simialis. 



The area striata is surrounded by two peripheral concentric bands an inner, 

 which may be called area parastriata, and an outer, the area peristriata. Sulci 

 develop along the boundary lines of each of these areas ; and those which indicate 

 the superior and inferior limits of the peripheral band (i.e. peristriate area" 



