STRUCTURE OF THE LOBES OF THE CEREBRUM. 379 



fornicatus ; the external and at the same time lower limb of the 

 angle, on the other hand, runs, as the external convolution of 

 the olfactory lobe, into the temporal extremity of the gyrus 

 fornicatus (the hooked convolution, gyrus uncinatus). Between 

 the two limbs the inferior surface of the corpus striatum, in- 

 vested by a thin layer of cortical substance, and known as the 

 lamina perforata anterior, forms the area of the olfactory 

 triangle. 



Inasmuch as, after the removal of the olfactory lobe, the 

 form of the internal surface of the hemispheres is essentially 

 that of a semi-circle open in front, the fundamentally arched 

 form of the latter becomes intelligible. 



This fundamentally arched form results also in another 

 mode from the external shield-like and unbroken surface of the 

 vesicle of the hemispheres. A portion of the convex wall of 

 the vesicle of the hemisphere coalesces with the external 

 surface of a ganglion that projects from the stem or trunk 

 of the brain into its cavity, the lenticular nucleus (corpus 

 striatum externum, fig. 233, L), from which it results that 

 the surface of this coalescence constitutes the only place at 

 which the ganglionic masses and the vesicles of the hemi- 

 spheres are in direct contact, without medullary union by 

 means of the projection system (see figs. 243, 244, 245). At 

 this point of coalescence the wall of the hemisphere remains 

 at its thinnest, and being therefore below the level of the sur- 

 rounding parts, sinks into the Sylvian fissure. The adhesion 

 itself begins at the anterior border of the convexity of the 

 hemispheres, close to that segment of the median ring which, 

 forming the outer olfactory convolution, appears in Man as 

 an intercalated stria of the olfactory triangular area ; it then 

 extends backwards and upwards somewhat beyond the half of 

 the outer surface. This fissure of Sylvius (which, when the 

 elevation of the lenticular nucleus is low, is a mere slit) repre- 

 sents the outer lumen of that convexity of the hemisphere open 

 anteriorly, which (fig. 233) surrounds the point of its coalescence 

 with the lenticular nucleus. The convexity is divided into 

 an upper segment, the frontal lobe (F) ; a lower segment, the 

 temporal lobe (T] ; and a posterior apical portion, the occipital 

 lobe (0). 



