Ramón y Cajal's 'lenticular tract' represents infrasubthalamic pyramidal collaterals targeting mesodiencephalic centers: an obscure misunderstood aspect of the motor pathway clarified by Allen Mouse Brain Connectivity data.

Ramón y Cajal repeatedly described a diencephalic tegmental tract in mammals, which he wrongly identified as corresponding to Forel's lenticular tract, while insisting it was formed by fasciculated infrasubthalamic collaterals of the pyramidal tract. These fibers circulated backwards between the zona incerta and the substantia nigra and were lost in the rubral neighborhood, so that their targets remained unknown. The modern literature does not register these pyramidal collaterals at all. In this report, anterograde axon-tracing experiments with motor cortex EGFP injections available at the Allen Mouse Brain Connectivity database were studied to assess whether Ramón y Cajal's collateral tract exists in the mouse and, in that case, examine the issue of its missing targets. All Allen motor cortex experiments showed straightforwardly the tract described by Ramón y Cajal, which he had lost at midcourse because it diverges into the mesodiencephalic alar plate. Moreover, it was observed to bifurcate as it enters the diencephalon into a deep medial component not distinguished by Ramón y Cajal, which targets diencephalic and mesencephalic preoculomotor cell populations and the deep ZI and Forel's field, and a thicker outer component, the part he described, which ascends through intermediate thalamic and pretectal alar structures (PIL, RETh, JcRt, and CoRt) until it ends densely on the paratectal bed nucleus of the brachium of the superior colliculus (BSC). The latter is frequently misinterpreted as a lateral part of the SC. Other midbrain targets included the rubral area, the MRt, a novel bilateral reticular endorubral field (ERF), and the preisthmic cuneiform nucleus (Cnf).