A retrothalamic system of collateral fibers from the cerebral peduncle

Ramón y Cajal described a sizeable bundle of collateral fibers from the cerebral peduncle and called it 'faisceau de Forel'. This bundle has been consistently ignored by recent authors, possibly because of the difficulty to visualize it in transversal section. It is described here as the fasciculus prerubralis, or FPR. It originates at the level of the transition from the nucleus subthalamicus to the substantia nigra. In sagittal-stained sections of the mouse brain stained with the rapid Golgi method it can be seen following an ascending and nearly vertical course. It arches caudalwards, and appears to terminate in the rostral tegmentum. In the cat, Nauta-stained material proved that the corticofugal fibers which contribute to this system originate preponderantly, if not exclusively, in the pericruciate gyrus. Following lesions in this cortical area one could observe, in addition to the degenerating fibers of this bundle, another system of degenerating direct cortico-tegmental fibers which coursed toward the rostral tegmentum. It is not clear whether this second degenerating system resulted from the encroachment of passing fibers in the subjacent internal capsule. The tegmental prerubral collateral fibers from the cerebral peduncle were observed in mouse, rat and cat. Possibly, they could represent a general feature of the mammalian nervous system.