Fornix system efferent projections in the squirrel monkey: An experimental degeneration study

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Abstract
The efferent projections of the fornix system have been studied in the squirrel monkey using modified silver staining techniques to delineate degenerating fibers and terminal boutons after inducing careful lesions of the subcallosal fornix and dorsal fornix.Subcallosal fornix projections, after supplying the medial and lateral septal nuclei, are traced via precommissural fibers to terminal degeneration in the nucleus of the diagonal band of Broca, nucleus accumbens, gyrus rectus, olfactory tubercle, medial and lateral preoptic areas, and medial hypothalamus. Degenerating precommissural subcallosal fornix fibers course with the medial forebrain bundle throughout the extent of the lateral hypothalamus, extending caudally to the medial mammillary nucleus with specific hypothalamic offsets to the tuberal region and nucleus intercalatus (“lateral cortico‐hypothalamic tract”). Subcallosal postcommissural fornix fibers are traced to terminal degeneration in the bed nucleus of the stria terminalis and medial preoptic and anterior hypothalamic structures (“medial cortico‐hypothalamic tract”); fornix column fibers disperse throughout the perifornical nucleus; these fibers more caudally supply nucleus intercalatus and medial mamillary nucleus. In the thalamus postcommissural fibers supply the nucleus reuniens, the paraventricular, anterior ventral, and lateral dorsal nuclei. In the mammillary nucleus a laminated radiation is described with some fibers extending caudally to the prerubral field.A “midline subcallosal stria,” derived from the subcallosal fornices, distributes fibers to the subfornical organ, the dorsal part of the medial septum, the precommissural fornix and especially the medial cortico‐hypothalamic tract.The dorsal fornix projection is predominantly pericallosal to the gyrus rectus and parolfactory area, with only a minor callosal‐penetrating contribution to pre‐ and postcommissural fibers.The composite projections of the hippocampal formation fornix system appear more extensive than those of the amygdala to basal forebrain, preoptic, and hypothalamic areas.