THE TELENCEPHALON. 1149 



separates the inferior temporal from the occipito-temporal gyrus. Although for 

 the greater part of its extent on the temporal lobe, it is not confined to this, but 

 continues backward into the occipital lobe which, therefore, claims it as one of its 

 furrows. The sulcus is rarely continuous, usually being broken by annectant gyri 

 into a posterior, a middle and an anterior segment. 



The occipito-temporal gyrus (gyrus fusiformis) is, as its names imply, a 

 fusiform tract belonging partly to the occipital and partly to the temporal lobe 

 (Fig. 989). Its two ends, in front and behind, are pointed and connected by a 

 broader intervening tract, which is commonly broken up by secondary furrows. 

 The temporal division of the gyrus, including approximately its anterior two-thirds, 

 is embraced between the converging collateral fissure mesially and the inferior 

 temporal sulcus laterally ; its conventional posterior limit is the line drawn from 

 the preoccipital notch to the isthmus of the limbic lobe, immediately beneath the 

 hind-end of the corpus callosum. 



The superior surface of the temporal lobe is directed towards the insula and 

 is therefore an opercular aspect. On separating the walls of the Sylvian fissure to 

 expose it, this buried surface of the temporal lobe often exhibits several shallow 

 transverse furrows and indistinct gyri, the deep aspect of the temporal pole being 

 similarly indented. 



Rolandic fissure Sulcus subdividing precentral lobule 



/ Cut surface of frontal lobe 



V /^ ^/ 



Sulcus centralis^ ^ T T" * Sulcus drcularfs 



Gvri breves 



Sulcus cetitralis insulse 



Gyrus longus Temporal Apex Limen 

 lobe, cut 



Island of Reil exposed after cutting away surrounding parts of right cerebral hemisphere. 



The Insula. The insula, or island of Reil, sometimes also called the central 

 lobe is in the human brain, entirely concealed within the Sylvian fissure by the 

 approximation of the overhanging opercula. The manner in which the latter are 

 developed from the wall surrounding the early Sylvian fossa has been described 

 (page in?) ; it remains here to note the chief features of this region in the adu 

 brain On examining the relations of the insula, as seen in frontal sections of 

 brain' (Fig. 967), it will be noted (a) that the shell of cortical gray matter cove 

 ing the sunken convolutions is directly continuous along the Sylvian fissure with t 

 covering the convolutions on the freely exposed parts of the hemisphere ; (*) 

 the insular cortex lies close to the underlying mass of gray matter, the len 

 division of the corpus striatum, a narrow tract of white matter, the external caps! 

 alone intervening. Since the corpus striatum is one of the earliest of the funda- 

 mental parts of the telencephalon to be developed, it is probable that its close pri- 

 mary relation to the surface of the hemisphere is largely responsible for the failure c 

 the overlying cortex to keep pace with the general expansion of the adjoining parts. 



When exposed by separation or removal of the surrounding opercula (1 

 QQI) the insula appears as a triangular convex field composed of a group t radi- 

 ating convolutions, whose broader ends lie above and pointed ones 



