The spinal afferents to the inferior olive were investigated in 15 rabbits and 17 cats, in which lesions of various extent were made at various levels of the spinal cord and the ensuing degeneration of the spino-olivary fibers was studied by the Marchi and Nauta methods.

The organization of the spino-olivary system of the rabbit is essentially similar to that of the cat, except for some details concerning the lateral cervical nucleus.

The spino-olivary fibers take origin chiefly in the posterior horn and possibly also in the intermediate zone at practically all levels of the spinal cord and ascend bilaterally through the anterior and the adjacent part of the anterolateral funiculus in close topographical relationship with the spinoreticular fibers and Gowers' fasciculus to terminate in the medial and the dorsal accessory olives. The termination in the olive is bilateral, but more marked contralateral to the nuclei of origin. The fibers from the lumbosacral cord are more numerous than those from cervicothoracic cord. While the majority of the fibers are considered to be collaterals chiefly of the spinoreticular and partly of the spinothalamic fibers, a considerable amount of true spino-olivary fibers appears to terminate in the caudal region of the medial and the dorsal accessory olives.

The lateral cervical nucleus cannot be considered the main source of the spino-olivary fibers.

Some details concerning the terminal distribution of the spino-olivary fibers are discussed.