THE TELENCEPHALON. 



1 149 



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. 



Fig. 991. 



Rolandic fissure 



Sulcus centralis 



Sulcus centralis insulse 



Sulcus subdividing preceiitral lobule 

 Cut surface of frontal lobe 



Sulcus circularis 



C-" 



Gyri breves 



Gyrus longus Temporal 

 lobe, cut 



Apex 



Litnen 



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 centi-al 

 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 1 137) ; it remains here to note the chief features of this region in the adult 

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

 brain (Fig. 967), it will be noted fa) that the shell of cortical gray matter cover- 

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

 covering the convolutions on the freely exposed parts of the hemisphere ; {b) that 

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

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

 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 of 

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



When exposed, by separation or removal of the surrounding opercula (Fig. 

 991), the insula appears as a triangular convex field composed of a group of radi- 

 ating convolutions, whose broader ends lie above and pointed ones below. The 



