ii68 



HUMAN ANATOMY. 



the definite dentate g>'rus, a part of it, greatly attenuated and reduced, retains its connection with 

 the anterior basal surface of the brain (later the anterior perforated substance) and follows the 

 upper surface of the corpus callosum, which likewise has extended backward, into the descend- 

 ing horn of the lateral ventricle. These parts — the gyrus subcallosus, the longitudinal striae, 

 the fasciola cinerea and the gyrus dentatus of the adult brain — constitute the supracallosal gyrus, 

 whose gray matter is an atrophic outlying part of the primary gyrus dentatus and whose con- 

 nections with the basal olfactory- centres are retained by the fibres of the longitudinal striae. 

 The fornix shares the displacement of its cortical area, the hippocampus, and is consequently 

 carried with the latter into the descending horn of the lateral ventricle. In this manner parts 

 which at first lay in proximity and were connected by short paths, become widely separated, 

 with corresponding lengthening of the fibre-tracts uniting them, as illustrated in the long 

 course of the fornix in the adult brain. Further, since the path of migration of the fornix 

 and associated structures of the inferior horn of the lateral ventricle describes a curve, 

 it follows that the relations of these parts become reversed, those originally lying 

 above, in regard to adjacent structures, within the descending horn being below and 

 vice versa. 



The posterior horn of the lateral ventricle (cornu posterius), much smaller 

 than either of the others, is an elongated diverticulum which curves backward from 



Fig. 1007. 



Superior frontal gyrus 



Longitudinal fissure-- 



Genu of corpus 



callosum- 



Lateral ventricle, 



anterior horn- 



^Middl2 frontal gyrus 



Inferior frontal gyrus 



Caudate nucleus, head 



Orbital gyri 

 Frontal section of brain passing through genu of corpus callosum. 



the body of the ventricle into the occipital lobe. In frontal sections (Fig. 1034) its 

 form is irregularly crescentic, the convexity of its outline including the roof and the 

 lateral wall and the concavity corresponding with the mesial wall and narrow floor. 

 Above and to the outer side, the horn is bounded by the arching fibres of the tape- 

 turn of the corpus callosum, lateral to which lies the important thalamo-occipital or 

 optic radiation (page 1123). The lower part of the mesial wall is modelled (Fig. 

 1000) by a narrow but well marked crescentic elevation, the calcar avis, also 

 called the hippocampus mhior, which is produced by the early invagination of the 

 wall of the hemisphere by the anterior part of the calcarine fissure. On the same 

 wall and just above the calcar avis, a second and broader, but less sharply 

 defined, elevation (bulbus cornu posterioris), marks the course of the fibres of the 

 forceps posterior as they encircle the parieto-occipital fissure in their journey to the 

 occipital lobe. 



