154 



THE CEREBRAL HEMISPHERES. 



THE CEKTTRAIi LOBE OR ISLAND OF REIL. This lobe is entirely 

 concealed in the adult human brain within the fissure of Sylvius, the margins of 

 which form opercula (see p. 142) which overlap the lobe. The central lobe corre- 

 sponds with the corpus striatum on the interior of the hemisphere (fig. 88), and 

 probably in consequence of its close connection with this mass of grey matter, which 

 appears early as a thickening of the lateral wall of the vesicle of the cerebral hemi- 

 sphere (vide Embryology, Vol. I., p. 69), this particular part of the hemisphere does 

 not keep pace with the general expansion which the hemisphere-vesicles undergo, 

 and hence in consequence of that expansion it becomes overlapped and concealed by 

 the rest of the hemisphere. On account of this early fixation and the close topo- 

 graphical relation between it and the largest of the basal ganglia, the central lobe 

 (along with the basal ganglia and the continuation of the peduncles between them) 

 is frequent!/ spoken of as the stem (German, Slammtheil) of the hemisphere, the 

 remainder of the cerebrum, which covers it in, being known collectively as the mantle. 



To see the island it is necessary to cut away the opercula (fig. 108). It then 

 appears as a triangular surface somewhat bulged outwards, the base of the triangle 



Fig. 108. ISLAND OF REIL EX- 

 POSED BY CUTTING AWAY THK 



UPPER OPERCULUM AND DRAW- 

 ING ASUNDER THE ORBITAL 

 AND TEMPORAL OPERCULA. 



(Eberstaller. ) 



s.c., sulcus centralis insulse ; 

 1, 2, 3, gyri breves ; 4, 5, gyms 

 longus ; s.li.a., s.H.ft., s.K.p., 

 anterior, superior, and posterior 

 limiting sulci ; I, limen insulse ; 

 F, orbital part of third frontal 

 gyrus; 2\, T z , first and second 

 temporal gyri; x.y, upper trans- 

 verse temporal gyri ; g. tr. i. , gyrus 

 transversus insulse, passing at a, 

 into the orbital part of the third 

 frontal ; b, connection of gyrus 

 longus insulse with apex of tem- 

 poral lobe ; m, short gyri on the 

 upper surface of the temporal pole. 



being directed upwards and the apex being at the vallecula Sylvii. This surface is 

 marked out by shallow sulci, which have a fan-like arrangement converging from the 

 base towards the apex of the triangle, into several straight gyri having a similar con- 

 vergent course. One of these sulci, which is deeper and appears earlier than the rest, 

 and is also more constant in lower Primates, has been termed the sulcus centralis 

 insulse (Guldberg) (fig. 108, s.c.), and this serves to subdivide the lobe into two 

 parts, aprecentral and postcentral lobule. Since the line of.direction of this sulcus 

 nearly corresponds with that of the fissure of Rolando in the mantle, the pre- and 

 post-central parts of the island similarly correspond to the frontal and parieto- 

 temporal lobes of the mantle ; and they are in fact in continuity with the parts 

 of those lobes which form the opercula. The island is, however, separated from 

 these adjacent parts by a sulcus (sulcus limitans insulse) which almost entirely 

 surrounds it. and which is itself formed of an anterior, a superior, and a posterior 

 part (fig. 108). The anterior and posterior parts of the limiting sulcus are, 

 however, deficient near the apex of the lobe, so that there here occurs a direct 

 continuity (by a small convolution, the gyrus transversus insulce of Eberstaller) 

 between the orbital part of the third frontal convolution as it dips round the margin 

 of the Sylvian fossa, and the precentral lobule on the one hand, and between the 

 linibic lobe and the extremity of the postcentral lobule on the other hand. 



