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PPORTUNITIES to observe the results of the isolated destruction of the "pyramidal tract" in man are rare. There are but few places in the nervous system where the corticospinal fibers are separated sufficiently from other systems to permit of their isolated destruction, either surgically or by disease. Extirpation of the pre-and postcentral cerebral cortex results in a destruction of the "pyramidal tract" but also destroys other fibers, both ascending and descending, thus producing a complex picture. Lesions of the internal capsule frequently destroy the corticospinal system but they also destroy descending pathways to the basal ganglia, the thalamus, the brain stem, the cerebellum, etc., as well as many ascending fibers. Lesions in the spinal cord, likewise, cannot be confined to the corticospinal fibers. Only in the pyramids of the medulla oblongata can truly isolated lesions of the "pyramidal tract" be made. Although division of the medullary pyramids has been made in animals (cat and monkey),~6,27 it has not been carried out in man and at the moment no occasion to do so seems likely to arise. In the cerebral peduncle the corticospinal fibers are found segregated from all ascending fibers 24 and from all descending pathways except those passing from the cerebral cortex to the brain stern and cerebellum. The corticospinal fibers from the precentral cortex (the classi