Cobalt injected into the right and left fasciculi retroflexes clarifies the organization of this pathway.

A cobaltic lysine complex was injected separately into the right and left fasciculi retroflexes of the frog. This tracing technique labeled, in a Golgi-like manner, the neurons which initiate the fasciculi retroflexes and revealed details of their morphological pattern. The fasciculi retroflexes originate from various neurons distributed in the diencephalon and mesencephalon, but their main source is the habenular nuclei. In the frog there are dorsal and ventral habenulae which are homologous to the medial and lateral habenulae, respectively, of mammals. In the frog the dorsal habenulae are strikingly asymmetric. Our study shows that the fasciculus retroflexus is composed of several compact separated bundles of fibers and that the fibers originating from the dorsal habenular nuclei project to the interpeduncular nucleus, while those originating from the ventral habenular nuclei project beyond the interpeduncular nucleus, but so far we have not been able to show the exact site of their termination. The labeling of cells within the interpeduncular nucleus raises the possibility that the habenulo-interpeduncular tract is reciprocal in function. The finds support our previously reported hypothesis on the theoretical interpretation of the functional circuitry of the frog habenulo-interpeduncular system.