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three or four limbs which run in various directions (see Figs.). All 

 deep sulci are bounded at either end by small gyri, which are them- 

 selves, naturally, marked off from the surface of the brain by furrows 

 of greater or less depth. It appeared to me that a very great deal of the 

 extraordinary complexity of the cerebral convolutions was due to the 

 complications produced by these small secondary gyri and sulci. 

 These are particularly well marked in the case of the furrows which 

 have been formed by the infolding of areas (2) primitively outspread 

 upon the cortex ("axial furrows"). For the slight extra heaping-up 

 of grey matter above the general surface of the brain which occurs 

 at all such special areas, causes small bounding furrows to arise at both 

 ends and on either side of the sulcus denoting the true extent of the 

 area, the major part of which has become lost from view in the walls 

 of the axial furrow — e. g. calcarine sulcus. For instance, at the caudal 

 extremity of the posterior calcarine sulcus there is a very well-marked 

 operculated gyrus, which is itself marked off from the surrounding 

 brain by a deep furrow — the sulcus lunatus of Elliot Smith. It might 

 be advisable to name this the gyrus lunatus instead of " operculum 

 occipitale", for though it is but a small convolution in itself on the 

 human brain it reaches a great size in the lower apes (9). Above 

 and below the calcarine sulcus are furrows which mark the limits of 

 the area striata, whilst the anterior calcarine sulcus bounds it in front. 



If we turn to the fissura or fossa parieto-occipitalis we find the 

 same arrangement. This furrow cuts deeply into the mesial surface 

 of the hemisphere and appears for a short distance upon its outer aspect 

 as the so-called external parieto-occipital sulcus. Eound the end of 

 the sulcus which presents externally is thrown a very well-marked 

 convolution, — the arcus parieto-occipitalis. This is itself separated 

 from the superior parietal field by a very deep definite furrow, the 

 sulcus paroccipitalis of Wilder [S). 



These are two concrete examples and it will now be shown how 

 the same phenomenon presents in the case of another sulcus — the 

 sulcus cinguli. Just behind the sulcus centralis (Eolandi) a furrow 

 cuts the upper border of the hemisphere and runs for a variable dis- 

 tance on the outer surface of the cerebrum. This is the hindmost up- 

 turned end of the sulcus cinguli (the sulcus calloso-marginalis of the 

 Old Terminology.) Round the end of this furrow is thrown a very 

 definite and constant gyrus, which may be called the arcus cinguli 

 (see Figs. 1 and 2, esp. Fig. 2. D). 



