913 THE XEItrK SYSTEM 



The lobes are: (1) the frontal; ('2) the parietal; (3) the temporal; (4) the occipital; 

 (5) the island of Reil or insula. The interlobar fissures are: (1) the sylvian; (2) 

 the central; (3) the occipital; (4) the calcarine; (5) the circuminsular. A series of 

 fissures demarcating the rhinencephalon from the pallium or cerebral mantle 

 proper will be considered at a later stage. 



The Interlobar Fissures. The Sylvian Fissure and Its Kami (fissura cerebri later- 

 alis [Sulvii]). This fissure is a well-marked cleft on the base and side of the 

 cerebral hemisphere. Traced laterad from the region of the anterior perforated 

 substance, it begins as a deep depression between the orbital surface of the frontal 

 lobe and the temporal pole, corresponding to the bony ridge formed by the lesser, 

 wing of the sphenoid and extending to the convex surface. This portion of the 

 fissure is termed the basisylvian fissure or vallecula sylvii, as far as the sylvian point.' 

 The svlvian point marks the conjunction of the main portion of the sylvian fissure 

 with its basisylvian part as well as one or two rami. These rami are (1) the 

 presylvian ramus and (2) the subsylvian ramus. 



The presylvian ramus 2 usually proceeds dorsad, slightly inclined frontad, for 

 a distance of 2 to 3 cm. into the subfrontal gyre. 



The subsylvian ramus (anterior horizontal limb) extends frontad for a distance 

 of 1.5 to 2 cm., parallel to the orbitofrontal (superciliary) margin. 



These two rami often spring as shorter branches from a common stem, and both 

 may be replaced by a single unbranched limb. 



The sylvian fissure proper is the most conspicuous part. It extends from the 

 sylvian point in a caudal direction, inclined slightly dorsad, 3 on the lateral surface 

 of the cerebrum for a distance averaging 6 cm. (2\ inches). It separates the 

 temporal lobe wholly from the frontal and partly from the parietal lobe. It usu- 

 ally ends in an upturned manner, in the parietal lobe, the change of direction 

 being oftener abrupt than gradual; this terminal piece receives the name of epi- 

 sylvian ramus. Occasionally a short ramus is sent ventrad into the supertemporal 

 gyre and is called the hyposylvian ramus. 



The sylvian fissure ranges in depth from 15 mm. or less at the presylvian point 

 to 25 or 30 mm. (1 inch to li inches) at the postsylvian point, correlative with 

 the contour of the island of Reil, which lies in its depths. If the lips of the sylvian 

 fissure be divaricated, the island of Reil is revealed as a cortical district, of tetra- 

 hedral form, which is normally completely concealed by overlapping portions of 

 the cerebral hemisphere called the opercula. These are four in number: (1) the 

 operculum proper, (2) the preoperculum, (3) the suboperculum, and (4) the post- 

 operculum. 



The operculum (frontoparietal operculum) is composed of the adjacent portions 

 of the ventral border of the frontal and parietal lobes, the sylvian fissure inter- 

 vening between it and the postoperculum, which is the overlapping part of the 

 temporal lobe. The preoperculum is a small triangular portion embraced by the 

 presylvian and subsylvian rami, and is also called the pars triangularis or Broca's 

 cap. The suboperculum (orbital operculum) is small, demarcated by the sub- 

 sylvian ramus, and, for the most part, on the orbital face of the frontal lobe, 

 projecting slightly over the frontal part of the island of Reil, with its margin 

 separated from the temporal pole by the basisylvian cleft. 



The overlapping opercula are demarcated from the island of Reil by the cir- 

 cuminsular fissure (sulcus circularis Reilii). 



Development of the Island of Reil and the Sylvian Cleft. The insular cortical 

 district is topographically correlative with the great gray ganglia 'of the cerebral 



1 So called in cranial topography. 



- Also called the anterior ascending limb. In BNA terms the name for this fissural branch is Itutnus nntcrior 

 ascendens fissurae cerebri lateralis. 



3 Its inclinntion to the horizontal plane is called the aylrinu untile, approximately 15 degrees. 



