Ascending projections of the lateral lemniscus in the cat and monkey

The ascending connections of the lateral lemniscus were studied in the cat and squirrel monkey ( Saimiri sciureus ). In both species, the central nucleus of the inferior colliculus receives a massive projection from the lateral lemniscus. Only a few lemniscal fibers were found to terminate in the external nucleus of the inferior colliculus. The commissure of the lateral lemniscus originates from the dorsal nucleus of the lateral lemniscus and projects to the contralateral dorsal nucleus and the contralateral central nucleus of the inferior colliculus. No lemniscal fibers were seen ascending in the inferior brachium or terminating in the principal division of the medial geniculate body. A bundle of fibers was observed, however, which passed medial to the inferior brachium and terminated in the magnocellular or internal division of the medial geniculate. The bundle degenerated after lesions confined to the lateral lemniscus and is probably identical with the central acoustic tract of earlier workers. Evidence is presented that the fibers of the bundle are of spinal origin. Since the lemniscal fibers which ascend to the thalamus appear to be non‐auditory, it is suggested that the inferior colliculus is an obligatory relay station in the classical auditory system and that the inferior brachium is the only ascending pathway of the system to project upon the thalamus.