The habenular nucleus of kittens was studied using the rapid Golgi and the Golgi-Kopsch silver impregnation methods. The neurons of the medial habenular nucleus (MH) were classified into two types. The type I neurons, the main constituents of the MH, had piriform cell bodies with a mean diameter of 12 mum by 18 mum and two to five primary dendrites; dendrites had many spines. The type II neurons (14 X 23 mum) were fusiform in shape and one to three primary dendrites arose from each pole of the cell bodies; dendrites had few spines. The axons of both types of neurons were traced into the fasciculus retroflexus Meynerti (FR), and intranuclear axon-collaterals arose from axons of the type I neurons. The neurons of the lateral habenular nucleus (LH) were divided into four groups. Type I, II and III neurons were projection neurons of large, medium and small size, respectively. The type I neurons (27 X 43 mum) with four to seven primary dendrites were located mainly in the rostral and ventral areas of the LH. The type II neurons (15 X 33 mum) with two to four primary dendrites, the main constituents of the LH, were distributed throughout the LH. The type III neurons (15 X 25 mum) with two primary dendrites emerging from each pole of the soma were localized to the mediocaudal areas of the LH. The vast majority of axons of these projection neurons passed ventrally or ventrocaudally to enter the FR; only a few axons of these neurons were traced into the stria medullaris thalami (SM). The type IV neurons (12 X 25 mum) were small cells with short axons, suggesting the existence of a neural circuitry intrinsic to the LH. Bundle formation and glomerular arrangement of dendrites were observed in the medium-sized LH neurons. The afferent fibers terminating within the MH coursed in the most part of the SM. These afferents were classified into medium-caliber type I and fine type II fibers; both of these fibers emitted many intranuclear collaterals. There were also observed many fibers of passage which ran between the SM and FR, or between the habenular commissure (HC) and FR. The afferent fibers to the LH were divided into three groups; afferents via the HC, ascending and descending afferents. Most of the descending afferents entered the LH through the SM; some of them traversed the LH to join the FR or HC, or to extend to the pretectal region. The vast majority of the ascending afferents entered the LH via the FR; some of them extended rostrally to enter the SM. Some fibers in the HC also terminated within the LH. In addition to these afferents, many fibers of passage were seen to run through the LH and to bridge over between the SM and FR, or between the SM and HC.

The subthalamic nucleus (ST) of kittens was studied by means of the rapid Golgi silver impregnation method. The neurons of the ST were classified into three types. Type I neurons, the main constituents of the ST, had oval or polygonal cell bodies with a mean diameter of 26 micrometer by 36 micrometer and four to six primary dendrites. Type II neurons had multipolar or polygonal cell bodies, which measured an average 31 micrometer by 43 micrometer and emitted four to seven primary dendrites. The cell bodies of the type III neurons were polygonal in shape, measured 23 micrometer by 26 micrometer in average and emitted four to six primary dendrites. The dendrite bundle and the dendrite pallisade were observed. Frequently dendrites of the ST extended into the cerebral peduncle (CP), and even cell bodies of some ST neurons were located within the CP. All of the parent axons of the ST neurons coursed rostrally, although intra- and extra-nuclear axon-collaterals arising from the ST neurons travelled rostrally, caudally or caudomedially. The afferent fibers to the ST were divided into three groups; afferents via the Meynert's commissure (MC), decending and ascending afferents. The MC fibers, which ran across the CP, gave terminals to the ST. The descending afferents were axon-collaterals of fibers descending in the CP and those of fibers running through the ST. The ascending afferents were also axon-collaterals arising from ascending fibers in the CP. In addition to these afferents, many descending and ascending fibers of passage ran through the ST without emitting axon-collaterals.

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