Some intrinsic connections of the hippocampus in the rat: An experimental analysis

Abstract Some internal pathways of the hippocampus were mapped in adult rats using Fink‐Heimer silver impregnation methods for the demonstration of anterograde axon degeneration. Cases with lesions of regio superior of the Ammon's horn showed that regio superior projects onto the subiculum by way of fibers in the alveus. The termination is most dense in the portion of the subiculum near the presubiculum. The fibers exhibit an orderly arrangement, in that the dorso‐ventral level of the subicular degeneration depends on the level of the regio‐superior lesion. No evidence of connections from regio superior to regio inferior and area dentata appeared in this study. The terminal fields of ipsilateral fibers from regio inferior to regio inferior and superior are pervaded by commissural fibers. This precludes selective destruction of the ipsilateral fibers in normal rats. In order to investigate these latter ipsilateral fibers, special animals were used in which the commissures had been transsected at the age of eight days. Secondary lesions were made in the hippocampus of the adults when no stainable commissural degeneration was demonstrable. The results obtained from these animals were the following. Lesions of regio inferior cause degeneration both in this subfield and in regio superior. The fibers terminate in the stratum oriens and radiatum, and not in the stratum lacunosum‐moleculare or on pyramidal cell bodies. The total spread of fibers along the longitudinal axis of the hippocampus is 4–5 mm, being slightly less in the regio inferior than in the regio superior. When the lesion involves only the dentate area and the adjacent part of the regio inferior, the degeneration in the regio superior is most pronounced superficially in the stratum radiatum and is absent from the stratum oriens. The fibers remaining within the regio inferior have a special mode of termination: dorsal to the lesion, degeneration is present in the deeper half of the stratum radiatum and in the stratum oriens; ventral to the lesion, degeneration is found predominantly in the superficial one half of the stratum radiatum.