THE TELENCEPHALON. n6 7 



the under surface of the uncus and then winds over the inner aspect of the latter, from 

 within outwards, as a narrow grayish band, the frenulum of Giacomini, which 

 continuing upon the upper surface of the uncus, for a short distance passes slightly- 

 backward and disappears (Fig. 1006). 



Followed backward, the gyrus dentatus accompanies the fimbria towards the 

 splenium, at the lower border of which the two structures part company, the fimbria 

 passing to the under side of the corpus callosum, whilst the gyrus dentatus, losing its 

 corrugations and becoming a smooth band, known as the fasciola cinerea, bends 

 backward and curves around the splenium (Fig. 992) to spread out over the upper 

 surface of the corpus callosum as the thin atrophic sheet of gray matter, the 

 induseum griseum in which are embedded the fibre-strands of the longitudinal 

 striae (page 1156). The structure of the gyrus dentatus is described with that of 

 other parts of the cerebral cortex (page 1182). 



FIG. 1006. 



-Splenium of corpus callosum 



-Uncus 

 Frenr 1 nm of Giacomini 



Fasciola cinerea Gyrus hippocampi \ Collateral fissure 



Gyrus dentatus 



Part of left gyrus hippocampi has been cut away to expose gyrus dentatus, which is seen continuing as 



frenulum of Giacomini over uncus. 



The fornix is to be regarded as the chief fibre-tract connecting the olfactory cortex, situated 

 within the uncus and the hippocampus, with the thalamus. An explanation of its remarkable 

 course as seen in the adult brain, is found in the changes which affect the position of the hippo- 

 campus during development. Reference to Figs. 1030, 1032, will recall the origin of the hemi- 

 sphere (pallium) as an outgrowth from the end-brain, and, further, that the hemisphere in man 

 early covers in the thalamus and other parts of the diencephalon and the mid-brain. For a time 

 the thalamus is connected with the hemisphere by means of only the thin recurved under and 

 inner wall of the pallium, the bulky tracts of white matter in which it is later embedded being 

 for a time wanting. This same independence is retained by the thalamus, even in the adult 

 condition, on its upper and posterior aspects, where the excessively thinned out ventricular wall 

 alone forms the partition between the ventricle and the exterior, and where the thalamus is over- 

 laid by, but not in contact with, the hemisphere. On breaking through this partition, as after 

 removal of the velum interpositum, the thalamus may be directly reached by passing beneath 

 the splenium. When a definite mesial surface of the hemisphere becomes developed, an area 

 along the inferior margin of this aspect becomes marked off by two primary grooves, which are 

 the early choroidal fissure below and the hippocampal fissure above. The area so defined is 

 the primary gyrus dentatus. This tract of gray matter is connected with the thalamus by the 

 fornix, which reaches the thalamus around the front end of the choroidal fissure. In many 

 animals, as in the rabbit, a similar relation is permanently retained, the dentate gyrus, or its 

 equivalent, the hippocampus, being united with the thalamus by a fornix-tract which sweeps from 

 the lower and posterior part of the pallium (hippocampus) over the roof of the third ventricle 

 forward and downward to the basal surface of the brain (mammillary body) and thence by 

 the bundle of Vicq cT Azyr to the thalamus. These primary relations are changed by the future 

 expansion of the hemisphere, which grows not only upward and backward, but also downward 

 to form the temporal lobe, in consequence of which the dentate gyrus and the fornix, and likewise 

 the choroid plexus and its fissure, are carried backward, downward and forward around the 

 thalamus into the temporal lobe, where they lie on the mesial wall of the descending horn of the 

 lateral ventricle which has coincidently been formed. Whilst in this manner the chief mass of the 

 primary gyrus dentatus is carried into the temporal lobe, where it becomes the hippocampus and 



