The author attempts to define the primary afferent centers in the brain-stem in terms of the known afferent centers in the spinal cord. The spinal root of the trigeminal nerve is homologized with Lissauer''s tract. The primary afferent centers for the relay of discriminative tactile and cognitive proprioceptive sensibility from the body are found in the nuclei of the dorsal funiculi. A nucleus of similar structure transmits stimuli coming through the trigeminal nerve. The author describes other nuclei lying in the same relative position with respect to the corpus restiforme that are probably concerned with the transmission of stimuli from the 7th, 9th and 10th nerves. The sensory nucleus of the facial nerve was found well developed in the elephant. The relay centers for mucosal sensibility are found in the tractus solitarius, a continuation of the pars intermedia of the spinal cord. Two nuclei lying close to the fasciculus solitarius and at the level of entry of the facial and glossopharyngeal nerves are considered to be the gustatory nuclei. In fetal specimens, prepared by the author''s special silver impregnation method, these nuclei are very evident, and can also be identified in Weigert preparations of the adult brain. Proprioceptive relay centers corresponding to Clarke''s column are found in 2 locations: the descending and ascending roots of the vestibular nerve, and the mesencephalic root of the trigeminal nerve. The descending root can be followed directly into Clarke''s column, and seems to receive fibers not only from the 8th nerve but also from the 9th and 10th. In this connection the author expresses his belief, supported by some evidence, that the 8th nerve is merely a highly specialized proprioceptive division of the 7th nerve, and that it has usurped functions previously carried out by the 5th and 10th nerves. After exenteration of the orbit in the cat a striking reduction occurred in the number of globoid cells of the nucleus mesencephalica trigemini on the operated side at the level of the nucleus of nerve III, while enucleations of the eyeball produced no recognizable changes. Although this nucleus seems to relay kinesthetic impressions from the muscles of mastication and from the extraocular muscles, these nerve cells are primary neurons with peripheral prolongations. The relay center corresponding directly with Clarke''s column was not identified.