836 



THE NERVOUS SYSTEM 



tally backwards and upwards to terminate in the supra-marginal gyrus of the parietal 

 lobe. 



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

 dividing the inferior gyrus of the frontal lobe. 



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

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

 superciliary border. 



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

 lapping or opercular portions of the adjacent pallium into (a) the temporal operculum, 

 which lies below the posterior ramus; (6) the fronto-parietal operculum, or operculum 

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

 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 central lobe or insula. The insula or island of Reil is a triangular area of the 



OPERCULU.V 



FIG. 623. THE INSULA WITH ITS GYRI AND SULCI. 



CYI1I HRKVKK i 



GYRUS LONG US ) 

 I 



Of IKSULA 



CIRCULAR SULCVS 



^ 

 N. 



, \ 7VM.VX1-KK.VK 



-\~^. y TKXPORAL 

 6- YKI 



I / 

 I/ 



ORBITAL GYRI 



CENTRAL SULCCS OF INSULA 



KITKRIUR TEMPORAL GYRUS 



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

 Of these, the temporal operculum overlaps the lobe to a greater extent than either the 

 frontal or parietal. More than half of it may, therefore, be exposed by gently press- 

 ing away the temporal lobe. The central lobe or island corresponds to the broad 

 floor of the Sylvian fossa of the embryonic brain. In the developed condition its 

 surface is convex lateral wards and is itself folded into gyri. The apex of the triangle 

 appears upon the basal surface of the hemisphere*, and is the only portion which may 

 be seen without disturbing the specimen. It appears as the end of a small gyrus 

 under the temporal pole, and in close relation with the anterior perforated substance 

 and the vallecula Sylvii, and is known as the limen of the insula. In the folding 

 process by which the opercula accomplish the overlapping and enclosing of the 

 island, there results a deep sulcus which 'surrounds its entire area except at the 

 limen insula!. This is known as the circular sulcus, or limiting sulcus of Reil. The 

 gyri (and sulci) of the island radiate from the apex of the triangle. The central 

 sulcus of the island is the deepest. It runs from below backwards and upwards, 



