174 Brain Research, 102 (197()) t, 7~', );-;il Elsevier Scientilic Publishing Company, Amsterdam Printed in The Nethcliai~ds The tractus infundibuli and other afferents to the parahippocampal region of the pigeon LARRY I. BENOWITZ* AND HARVEY J. KARTEN Department of Psychology, MIT, Cambridge, Mass. 02139 and Departments o]Psychiatry and Anatom- ical Sciences, State University (~[ New York, Stony Brook, N. 11. 11790 ( U.S.A.J (Accepted October 8th, 1975) While recent anatomical studies have contributed greatly to our understanding of the avian telencephalonU,l~,t4,1s, ?2, little is yet known about the dorsomedial forebrain, an area considered to be homologous with the mammalian hippocampus and related structures ~,~,4,8,16. We have now begun to examine the connections of this region in the pigeon in order to shed some light on the comparative anatomy of the hippocampal complex. Our first step was to establish a homologue to the fornix longus, which would then allow us to delineate the avian equivalents of structures known to be connected by that fiber system in mammals. Although the fornix longus has actually been dem- onstrated in birds, one proposed candidate has been the tractus infundibuli (IN) l, a distinct bundle of large, myelinated axons which courses through the ventral hypo- thalamus 7. The present study has focused on examining the origins and terminal projections of IN, along with some other neural associations of the dorsomedial telencephalon. To first determine the polarity of the IN and its locus of termination, lesions were placed stereotaxically la in the dorsal telencephalon, septum, dorsal anterior thalamus, or at the ventral portion of the di-mesencephalic junction. The animals were sacrificed 2 or 4 days later, perfused with saline and formalin, and prepared according to the method of Fink and Heimer ~ for degenerating boutons. In contrast to earlier descriptions 1,7, we found the IN to be an ascending system which projects to the dorsomedial forebrain. Following ventral tegmental lesions (Fig. 1), large degenerating axons of IN were seen to cross in the supramammiltary decussation (retroinfundibular commissure), travel anterolaterally until reaching the so-called nucleus mammillaris lateralis, then group into fascicles forming a distinct bundle ventral to the medial forebrain bundle. Rostral to the anterior commissure, at the level of the nucleus of the diagonal band of Broca, the fibers turn dorsally and * Current address: Department of Biological Chemistry, Harvard Medical School, McLean Hospital, Belmont, Mass. 02178, U.S.A.