856 



THE NERVOUS SYSTEM 



in a depression in the anterior perforated substance, the vallecula Sylvii, and passes 

 forward and upward between and separating the temporal pole and the super- 

 ciliary border of the frontal lobe. It corresponds in direction with the posterior 

 border of the lesser wing of the sphenoid bone, which projects backward into it, 

 and it contains the middle cerebral artery, the Sylvian vein, and the sinus alse 

 parvse. It appears on the upper surface at a point known in cranial topography 

 as the Sylvian point, where it divides into its three main branches: — 



(1) The posterior ramus is the linear continuation of the fissure, and runs 

 horizontally backward and upward to terminate in the supra-marginal gyrus of 

 the parietal lobe. 



(2) The anterior ascending ramus passes upward for about 10 mm., sub- 

 dividing the inferior gyrus of the frontal lobe. 



(3) The anterior horizontal ramus passes forward from the stem of the fissure 

 about 10 mm., and likewise into the inferior frontal gyrus, but parallel with the 

 superciliary border. 



Fig. 675. — The Insula with its Gtri and Sulci. (Shown by widely separating the opercula.) 



Gyri breves ] 



Operculum I ^ < of insula 



\ I (jyrus longus J 



\ // ' 



\ , / ' 



Circular sulcus 



Transverse 

 — .? temporal 

 gyri 



/ / 

 Orbital gyri 



Central sulcus of insula 



Superior temporal gyrus 



These branches, together with certain smaller collateral twigs, divide the over- 

 lapping or opercular portions of the adjacent pallium into (a) the temporal opercu- 

 lum, whicli lies below the posterior ramus; (b) the fronto-parietal operculum, or 

 operculum proper, which lies above and behind the anterior ascending ramus; (c) 

 the frojital operculum, between the latter and the anterior horizontal ramus; {d) 

 and the orbital operculum, below the anterior horizontal ramus. Collectively the 

 opercula are known as the opercula of the insula. 



The insula (central lobe). — The insula or island of Reil is a triangular area of 

 the cerebral cortex lying in the floor of the lateral fissure, and concealed by the 

 opercula. Of these, the temporal operculum overlaps the insula to a greater 

 extent than either the frontal or parietal. More than half of it may, therefore, 

 be exposed, by gently pressing away the temporal lobe. The insula corresponds 

 to the broad floor of the Sylvian fossa of the embryonic brain. In the developed 

 condition its surface is convex latcralward and is itself folded into gyri. The apex 

 of the triangle ajjpears upon the l)as;il surface of the homis])here, and is the only 

 portion which may be seen without disturbing the specimen. The apex appears 

 as the end of a small gyrus under the temporal pole, and in close relation with the 



