The anterior cochlear nucleus was examined with a view to describing its subdivisions. The brains of 29 adult rats were paraffin embedded, sliced at 16 μ and impregnated according to the protargol method of Bodian. From examination of this material the nucleus was divided into three major regions.
Region III contained round cells with short dendrites which received as synaptic endings two or three bulbs of Held and many terminal rings. The cells of the region gave rise to small diameter (less than 1 μ) nerve fibers which entered the trapezoid body. This region appeared to be identical with region III of Lorente de No.
Region II occupied the ventral one third of the nucleus and contained cells of various types, Synaptic endings in this region consisted principally of terminal rings of various sizes and complexities. Nerve fibers which originated in this region were between 1.0 μ and 2.2 μ in diameter and entered the trapezoid body. The region did not conform to region II of Lorente de No.
Region I occupied the dorsal two thirds of the nucleus and contained medium and small cells. Synaptic endings consisted of pale, small bulbs of Held. This region differed from region I of Lorente de No.