The efferent connections of the medial (MHb) and lateral (LHb) habenular nuclei in the rat were demonstrated autoradiographically following small injections of tritiated amino acids localized within various parts of the habenular complex. Comparison of individual cases led to the following conclusions. MHb efferents form the core portion of the fasciculus retroflexus and pass to the interpeduncular nucleus (IP) in which they terminate in a topographic pattern that refects 90 degrees rotations such that dorsal MHb projects to lateral IP, medial MHb to ventral, and lateral MHb to dorsal IP. Most MHb fibers cross in the interpeduncular necleus in the "figure 8" pattern described by Cajal, and terminate throughout the width of IP with only moderate preference for the ipsilateral side. However, the most dorsal part of MHb projects almost exclusively to the most lateral IP zone in a cluster pattern that is particularly dense on the ipsilateral side. The MHb appears to have no other significant projections, but very sparse MHb fibers may pass to the supracommissural septum and to the median raphe nucleus. Except for some fibers passing ventrally into the mediodorsal nucleus, all of the LHb efferents enter the fasciculus retroflexus and compose the mantle portion of the bundle. No LHb projections follow the stria medullaris. In the ventral tegmental area LHb efferents become organized into groups that disperse in several directions: (a) Rostrally directed fibers follow the medial forebrain bundle to the lateral, posterior and dorsomedial hypothalamic nuclei, ventromedial thalamic nucleus, lateral preoptic area, substantia innominata and ventrolateral septum. (b) Fibers turning laterally distribute to the substantia nigra, pars compacta (SNC); a small number continue through SNC to adjacent tegmentum. (c) The largest contingent of LHb efferents passes dorsocaudally into paramedian midbrain regions including median and dorsal raphe nuclei, and to adjacent tegmental reticular formation. Sparse addition LHb projections pass to the pretectal area, superior colliculus, nucleus reticularis tegmenti pontis, parabrachial nuclei and locus coeruleus. No LHb projections appear to involve the interpeduncular nucleus. All of these connections are in varying degree bilateral, with decussations in the supramammillary region, ventral tegmental area and median raphe nucleus. On the basis of differential afferent and efferent connections, the LHb can be divided into a medial (M-LHb) and a lateral (L-LHb) portion. The M-LHb, receiving most of its afferents from limbic regions and only few from globus pallidus, projects mainly to the raphe nuclei, while L-LHb, afferented mainly by globus pallidus and in lesser degree by the limbic forebrain, projects predominantly to a large region of reticular formation alongside the median raphe nucleus. Both M-LHb and L-LHb, however, project to SNC. The reported data are discussed in correlation with recent histochemical findings.

Retinal fibers in both the pigeon and owl terminate in a multinucleate complex of the dorsal thalamus, including the nuclei lateralis anterior, dorsolateralis-anterior, dorsolateralis anterior, pars lateralis et pars magnocellularis, and collectively designated the nucleus opticus principalis thalami (OPT) I Efferent projections of OPT were traced with the Fink-Heimer method into the ipsilateral lateral forebrain bundle, and via the dorsal supraoptic decussation, into the contralateral lateral forebrain bundle. OPT projections terminate within an elevation. or "Wulst" on the dorsum of the telencephalon. The Wulst is a multilaminate structure containing a deep lying layer of large cells the hyperstriatum dorsale (HD), a dispersed cell layer -the hyperstriatum intercalatus suprema (HISm), a granule cell layer or nucleus intercalatus hyperstriatum accessorium (IHA), an overlying hyperstriatum accessorium (HA) consisting of a broad layer of medium sized neurons, and an overlying fibro-molecular layer. Each of these laminae are particularly well developed in the owl, where the granule cell layer is divisible into inner and outer bands (IHAex and IHAint). The projections of OPT terminate in the HD, HISm and IHA. A homotopic projection was also found in the contralateral Wulst. The pattern of termination was similar in both the pigeon and the owl, though the pattern of distribution was more apparent in the owl with its massive OPT and Wulst. Medial, nonvisual, thalamic cell groups in the pigeon (nuclei dorsolateralis pars medialis and dorsomedialis anterior) also project bilaterally upon the U'ulst, but terminate in a more medial, nonvisual and cytologically different, portion of HD. The projections of the medial thalamic nuclei did not overlap with those of OPT and appear to be a separate functional system of still undetermined nature.Efferent axons of the "visual Wulst" of the pigeon and owl project upon the ipsilateral lateral hyperstriatum ventrale, neostriatum and upon the peri-ectostriatal belt (Karten and Hodos, '70). Extratelencephalic projections via the septomesencephalic tract (TSM) terminate in OPT, the internal lamina of the ventral lateral geniculate nucleus (LGv), pretectal nuclei and optic tectum. A small contingent of fibers of the TSM cross to the opposite side in the dorsal supraoptic decussation, to terminate in LGv, and, in the owl, in the contralateral ventromedial tectum. Dense terminal degeneration has also been observed in the deeper layers of the ipsilateral optic tectum of pigeon whereas in the owl the projection of the Wulst also extends to the more superficial layers of the tectum, and appears to be topographically arranged.The numerous similarities between the system described above and the genicdostriate visual pathways of mammals seems apparent. These findings clearly indicate that the geniculo-striate type of system may attain elaborate degrees of development in nonmammalian as well as mammalian brains.

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