Projections of the hippocampus to the septal area in the squirrel monkey.

An experiment was performed to determine the projections of the anterior, central, and posterior regions of hippocampus to the spetum in the squirrel monkey. Lesions were placed in each of these regions of hippocampus in 18 animals. Survival times varied from 5 to 7 days. Tissues were stained principally by the Fink-Heimer I method for identification of degenerating axons and their terminals. Following lesions which destroyed one or more fields of the anterior third of hippocampus degenerating axons were traced through the fimbria and fornix to the lateral septal area where dense quantities of terminal degeneration were observed. Smaller quantities of terminal degeneration were noted in the nucleus accumbens as well. After lesions of the middle third of hippocampus terminal degeneration was observed in an intermediolateral position within the septum. Following lesions of the posterior third of hippocampus degenerating terminals were observed in the area of the medial septum. All regions of hippocampus send fibers to the nucleus of the diagonal band and to the olfactory tubercle. These findings indicate that in the primate the projection to the septum from the hippocampus is topographically organized along its longitudinal axis in a manner that links the anterior hippocampus to the lateral septum and the posterior hippocampus to the medial septum.