The new operation of intramedullary tractotomy (Sjöqvist) has reawakened an interest in the anatomic arrangement and physiologic functions of the fibers composing the descending tract of the trigeminal nerve. Current ideas on these matters have been derived largely from clinicopathologic observations in cases of thrombosis of the posterior inferior cerebellar artery and of syringobulbia in which the intramedullary trigeminal fibers were involved in the pathologic process. Lesions of this character, because of their irregular size and intensity of destruction, have obvious disadvantages when a study of the clinical effects relating to impairment of a single structure or of an isolated tract is desired. Surgical transection of the descending tract of the trigeminal nerve in man, performed by means of a small controlled incision into the tuberculum cinereum, obviates many of the former difficulties and allows for the first time correlation of sensory changes with a small lesion accurately placed in