The morphological significance of the cerebellum is first outlined. Developed as it is from the region of termination of the vestibular nerves, the cerebellum constitutes a group of reflex centers for the correlation and elaboration of proprioceptive impulses from the labyrinths and body muscles; aroused in this way it discharges influences which augment or inhibit the postural tone of the muscles of the body. Ingvar''s views on cerebellar morphology are reviewed; he divides the organ transversely into the lobus anterior, lobus medius and lobus posterior; these subdivisions he considers more significant than the older "vermis" and "hemispheres." The regions of termination of the vestibulocerebellar, spinocerebellar and cerebro-pontine fibers are described and depicted. Effects of cerebellar removal are but slight in fishes, somewhat greater in amphibians and, according to recent work, still more marked in reptiles. The decerebellate pigeon is markedly ataxic, though, as in the mammal, consciousness and sensibility are unchanged. Luciani''s classical researches on cerebellar function are reviewed. The "dynamic phenomena" of his 1st stage are envisaged as "release phenomena," the inhibitory control of the cerebellum having been abolished on its removal. The cerebellum is not concerned fundamentally in the coordination of movements like those of progression, although it refines and grades such activities by its influences on postural tone. Hypotonia as a symptom of cerebellar removal is denied by de Barenne, though always noted by Holmes following gunshot wounds of the cerebellum. Holmes also demonstrated asthenia, astasia, asynergia, adiadocho-kinesis, and nystagmus. Complete acceptance of the Bolk-Rynberk theory of localization appears impossible, although fairly constant symptoms in neck and foreleg result from lesions of lobulus simplex and crus 1 respectively. There appears to be a "prevalence of representation" in certain locations; e.g., the arm is represented more anteriorly and the leg more posteriorly in the lobulus ansiformis, though the centers for both extremities are intermingled. The cerebellum has been found unessential for the "tonic labyrinthine reflexes," "tonic neck reflexes," and "righting reflexes" of Magnus and de Kleyn; it may, however, add elements of delicate modification and control to these activities. The cerebellar cortex is excitable. In conclusion: in the cerebellum are intricate reflex arcs, which, in response to streams of proprioceptive impulses from the labyrinths and muscles, emit in finely graded amounts impulses capable of influencing postural tone in augmentation or inhibition.