In the lateral region of the medulla, between the inferior olive and the descending trigeminal root, lies a fairly large group of cells about which considerable confusion exists. This nucleus has been described under various names, including the nucleus funiculi lateralis, nucleus reticularis lateralis and nucleus lateralis medullae. Most observers agree that this nucleus is a part of the afferent system of the cerebellum. Impelled by studies on the cerebellum and its connections initiated by Ferraro and one of us (S. E. B.), we undertook to study this nucleus carefully and to define as precisely as possible its architecture and its afferent and efferent connections. The location, structure and connections of this nucleus of the medulla have been discussed by Tilney and Riley,1Strong,2Cajal,3Jacobsohn,4Marburg,5Brun,6Ranson,7Weed,8Ariëns Kappers, Huber and Crosby9and others. A careful perusal of