HIPPOCAMPUS AND GYRUS DENTATUS.
The hippocampus, or cornu Ammonis, and the gyrus dentatus represent two spe cial convolutions. On following the gyms hippocampi dorsally, one reaches the subin culum, that constitutes that part of the hippocampal convolution in which gradually begins a change in the structure of the cerebral cortex, leading finally to the typical structure of the hippocampus. The white substance splits into two layers : the one passes to the free surface of the hippocampus and constitutes the alveus, the other passes to the lateral wall and roof of the inferior horn of the lateral ventricle. The alveus is continuous with the fimbria. The uppermost layer of the gray substance, the substantia reticularis alba, corresponding to the molecular layer of the typical cortex, divides into a superficial and a deep stratum. The superficial layer adjoins the molecular layer of the gyrus dentatus and forms the lamina medullaris circumvoluta. The deep layer forms the stratum lacunosum, that arches around and embraces the lamina medullaris and ends in a recurved hook at the medial side of the cell-layer of the gyrus dentatus. Between the
lamina medullaris circumvoluta and the stratum lacunosum, lies the stratum moleculare. The pyramidal cells of the subinculum gradually collect into a single layer of cells as they approach the hippocampus. At first the arrangement of the cells is still irregular, towards the gyrus dentatus the cells form a single thick layer, but within the terminal sheet of the hippocampus they once more are irregularly disposed. In this way two special zones are produced, a deep layer of pyramidal cells, the stratum lucidum, and, between the latter and the stratum lacunosum, the stratum radiatum, so called on account of the traversing long primordial stems of the pyramidal cells. The layer of polymorphic cells is known as the stratum oriens. The gyrus dentatus exhibits three strata : the molec ular layer, the granule-layer or the stratum granulosum, and the layer of polymorphic cells. The relations between the strata of the modified cortex of the special convolutions and those of the typical cerebral cortex are shown in the following table and Fig. 117 :