Afferent and efferent fiber connections of the carp torus longitudinalis.

The efferent and afferent pathways of the carp torus longitudinalis were studied by means of degeneration and retrograde HRP methods. Efferent projections were only seen in the most superficial layer of the ipsilateral optic tectum (stratum fibrosum marginale). Afferent pathways to the torus longitudinalis were found to originate mainly in the valvula cerebelli. Degenerating fibers course in the tractus mesencephalocerebellaris posterior within the valvula, and join the tractus mesencephalocerebellaris anterior in the tegmentum. The fibers which ascend in the tract gradually invade the optic tectum through which they are distributed to the torus longitudinalis. The remaining fibers pass through the posterior commissure and terminate in the torus longitudinalis at the rostral end of the tract. Degenerating terminals were also seen in the torus longitudinalis when lesions were made in the optic tectum, tectal commissure, torus semicircularis, and in the area between the valvula and the corpus cerebelli. The possibility of projections from these areas is discussed depending upon the results of the retrograde HRP method.