GYRUS DENTATUS.
The gyrus dentatus constitutes a small modified cerebral cortex, which adjoins the lamina medullaris circumvoluta of the hippocampus and receives within its hilus the end of the hippocampus. The white substance of the gyrus dentatus is net directly applied to the layer of polymorphic cells, but is separated from the latter by the cortical forma tion, which corresponds to the region of the hippocampus bordering the gyrus dentatus. It follows, that the fibres coming from the gyrus dentatus break through the end of the hippocampus lying within the hilus, the alveus, therefore, representing the cortical white substance of both the hippocampus and the gyrus dentatus.
Passing from the fissura hippocampi towards the ventricle, the following strata are encountered : a. Molecular layer, bordering the lamina 1 medullaris of the hippocampus, b. Stratum granulosum, Gyrus dentatus.
c. Layer of polymorphic cells, d. Molecular layer, e. Layer of giant pyramidal cells, f. Layer of polymorphic cells, Hippocampus.
g. Alveus. The cells exhibited by the individual strata of the gyrus dentatus are the following: i. Molecular layer: a. Cells of Golgi II type, b. Aberrant granule-cells.
2. Stratum granulosum.
This layer is formed of the granule-cells, closely placed and disposed in several rows. The cells are modified pyramidal cells, distinguished by the absence of the basal dendrites and a primordial stem. The ascending dendrites end within the molecular layer, while the nerve-process passes through the layer of polymorphic cells, then through the molecular layer and the stratum of pyramidal cells of the hippocampus, and exhibits during its further course peculiar local thickenings, with small protruding out growths. The nerve-processes unite into a bundle and then end, after forming a
reticular plexus, around the cell-bodies of the large pyramidal cells and their dendrites. They establish relations, therefore, between the granule-cells and the giant pyramidal cells of the hippocampus, from which, in turn, the impulse may be conveyed to other pyramidal cells by the collaterals that pass to the stratum lacunosum.
3. Layer of polymorphic cells: a. Cells with ascending nerve-processes, ending within the granule-layer, b. Cells with ascending nerve-processes, passing to the alveus, c. Cells of Golgi II type.
As in the hippocampus, so here, among the fibres passing from the gyrus dentatus are those coming from the alveus and ending within the gyrus dentatus.
In its further course, the gyrus dentatus continues, as the induseum griseum, over the corpus callosum. The medial and lateral thickenings, the stria Lancisii and the taenia tecta, likewise display the character of cortex ; within the stria Lancisii a molecular 'layer with tangential fibres, a middle layer with fusiform cells and a deep layer are distinguishable.