GYRUS DENTATUS. 527 
callosal gyrus is separated from the marginal convolution by the calloso-marginal 
fissure, and behind this it is imperfectly marked off from the preecuneus by the 
_ post-limbic sulcus. The furrow which separates it from the corpus callosum is 
termed the callosal sulcus. 
The hippocampal convolution is bounded on the outer side by the anterior part 
of the collateral fissure, and in front of this by the incisura temporalis, which 
separates its hooked extremity, or uncus, from the temporal pole. On the inner side 
it 1s limited by the hippocampal or dentate fissure, whilst posteriorly it is divided 
into two parts by the anterior extremity of the calcarine fissure. Of these, the 
upper is the isthmus, which connects it with the callosal gyrus, whilst the lower 
portion brings it into continuity with the gyrus lingualis. The surface of the 
hippocampal convolution is covered by a white reticular stratum of fibres, termed 
the substantia reticularis alba. 
Gyrus Dentatus and the Fimbria.—If the dentate fissure, which lies along 
the inner side of the hippocampal convolution, be opened up, the gyrus dentatus 
and the fimbria, lying side by side, will be brought into view (Fig. 586, p. 518). 
The fimbria is simply a portion of the posterior pillar of the fornix prolonged 
into this region. It is a conspicuous band of white matter, which presents a 
prominent free border. In front it runs into the recurved extremity of the uncus, 
whilst, if traced backwards, it will be seen to curve upwards behind the posterior 
end of the optic thalamus and become continuous with the posterior pillar of the 
fornix below the hinder part of the corpus callosum. 
Choroid plexus in lateral 
Ve entricle 
ve) 
f “\_/Corpus callosum 
eo 
=< —Fornix 
a 
Caudate Be) y 
Thalamus (pulvinar) 
Occipital corticifugal tract to 
superior quadrige ‘minal bod 
ss Superior quadrigeminal body 
Optic radiation Corpus geniculatum externum 
Corpus geniculatum internum 
Ve = 
Caudate nucleus j Sylvian gray matter 
Inferior brachium 
)) ~ Lateral fillet 
Optic radiation 
Inferior longitudinal bundle 
~ Superior cerebellar peduncle 
> 
Tapetum " 
~ Cerebellum 
Descending horn of lateral ventricle~ 
Fimbria” 
Pons 
Gyrus dentatus ie th 
Dentate fissure 
Fria. 391.—CoroNAL SECTION THROUGH THE LEFT SIDE OF THE CEREBRUM, MESENCEPHALON, AND PONS, 
IN THE REGION OF THE PULVINAR OF THE THALAMUS AND THE CORPORA GENICULATA (Chimpanzee ; 
Weigert-Pal specimen). 
The gyrus dentatus is the free edge of gray matter which is placed between the 
fimbria and the deep part of the upper surface of the hippocampal convolution. 
The groove between it and the fimbria is termed the fimbrio-dentate sulcus, whilst 
the furrow between it and the hippocampal convolution forms the bottom of the 
dentate fissure. The dentate gyrus is notched along the margin, whilst its surface 
is scored by numerous parallel and closely-placed transverse grooves. It begins 
behind in the region of the splenium or thickened posterior margin of the corpus 
callosum, and is carried forwards into the cleft of the uncus. From this it emerges 
