The inferior olivary nucleus is a conspicuous cell group in the ventral portion of the medulla oblongata. Its comparative neurological aspects have been intensively investigated, some of its connections are known, but specific evidence for its functional significance has been disappointingly meager.
Three subdivisions of this nucleus have been distinguished : a medioventral accessory part, present in fishes ; a dorsal accessory portion, added in birds; and a main part which makes its appearance in birds and becomes progressively more prominent in the mammalian series. This main division attains huge proportions in the primate brain where, as its name implies, it constitutes the major portion of the olive (Ariens Kappers, Huber and Crosby, '36). As shown by Essick ('12) the cells of the olivary nucleus arise from the rhombic lip of the alar plate and exhibit in their embryologic development in higher vertebrates, the several stages just mentioned.
Some efferent connections from the inferior olive are distributed to the spinal cord (Helweg, 1888; Thalbitzer, 'lo), but the main outflow passes to the cerebellum (Yoda and Kalagiri, '41 ; Brodal, '40 ; Brouwer, '13), and in this respect the olive would appear to be a sort of bulbar pons. This olivo-cerebellar projection is a precisely oriented one, the accessory olivary parts sending connections to the anterior and posterior cerebellar lobes, and the main olive to the middle lobe, whose development it parallels.
Cortico-olivary connections from the frontal portion of the hemisphere have been identified (Mettler, '35), but in the matter of its afferent connections the olive would appear to be less exclusively related to the cerebral hemisphere than is the pons, for afferent olivary connections arise also from the rostra1 brain stem, forming what has lThesis submitted in partial fulfillment of the requirements for the degree of Master of Science.