Efferent connections of the hippocampus and adjacent regions in the pigeon.

In this investigation, the projections of the dorsomedial (viz. hippocampus) and adjacent dorsolateral cortical mantle (viz. parahippocampus) of the pigeon telencephalon were studied with the use of 3H-amino acid radioautography. The results indicate that the fibers which arise from these cortical areas project to the septum. The projection to this structure is topographically organized in a crude manner along the longitudinal axis of the hippocampal complex. Fibers arising from rostral levels of the dorsomedial hippocampal complex project principally to central portions of the precommissural septum and to the nucleus of the diagonal band. Cells situated in the rostral parahippocampal area project to the entire region of the commissural and precommissural septum. Fibers arising from the caudal aspect of the dorsomedial cortical mantle project ipsilaterally to the dorsolateral septum and bilaterally to central portions of the postcommissural septum. However, fibers arising from more lateral parts of cortex terminate ipsilaterally along the entire longitudinal extent of the septum. Other classical projections of the hippocampal formation in the mammal which innervate the hypothalamus and thalamus could not be identified in the present study. Thus, it appears that the projections arising from both the hippocampal formation and adjacent lateral cortices of the pigeon correspond entirely to the precommissural fornix of the mammal.