Summary: The neuronal organization of the habenular ganglion (HG) was studied in the red stingray using the rapid Golgi method. The HG was made up of the medial (MH) and lateral habenular nucleus (LH), and the former nucleus was further divided into a dorsal, intermediate and ventral subnucleus. Only one type of neurons were observed in the MH, while the LH was composed of two types of neurons. In the left HG cut at the rostrocaudal middle of the ganglion, the LH was located in the dorsolateral region, while the dorsal, intermediate and ventral subnuclei of the MH occupied the dorsomedial, intermediate and ventral portions of the ganglion, respectively. In contrast, the right ganglion seen at this level was composed exclusively of the MH, with the dorsal, intermediate and ventral subnuclei located in the dorsomedial, intermediate and ventral portions, respectively. In the caudal level of the left ganglion, each nucleus was seen almost in the same region as in the level of the rostrocaudal middle, however, three subnuclei of the MH fused with the same subnuclei of the opposite side. In the right ganglion at the caudal level, the LH appeared in the intermediate area The right LH was far smaller and was located more ventrocaudally than the left LH. On account of the LH, the intermediate subnucleus of the MH was divided into a dorsal and ventral part. The dorsal and ventral subnuclei of the MH remained in the same region as in the rostral level. Thus, the HG of the red stingray exhibited a striking left-right asymmetry, the most remarkable aspect of which was considered to be differences of the size, form and location of the LH between the left and right HG.The habenular ganglion (HG) is a conspicuous structure in the dorsomedial region of the diencephalon and is known to exist in all vertebrates. One of the most remarkable features of the HG is the structural left-right asymmetry observed in certainspecies (Ariens Kappers, Huber and Crosby 1960;Kuhlenbeck 1977). The selachian HG has been known to exhibit a striking left-right asymmetry and has been the subject of numerous investigations (Edinger

The intrinsic organization of the olfactory bulb (OB) was studied in the red stingray using the rapid Golgi method. The OB is horse shoe-shaped, surrounding the equator region of the nasal capsule. As seen in the sagittal sections, the OB is round with the long olfactory peduncle extending from the dorsocaudal region and the olfactory fibers in a thick bundle entering from the rostroventral aspect. Although not so distinct, the following areas are distinguished. A rostroventral ovoid area adjacent to the entrance of the olfactory fibers consists exclusively of the olfactory fibers running in various directions. Dorsocaudal to the olfactory fiber area is a wide crescent region containing thin bundles of olfactory fibers, olfactory glomeruli, mitral cells and a few disseminated granule cells. A narrow crescent area made up of scattered granule cells is located dorsocaudally to the above wide crescent area. The outermost region consists of a fiber layer encapsulating the dorsal to caudal aspect of the OB. Thus, while the major constituents of the vertebrate OB are recognized, the lamination is very obscure.

The intranuclear course, distribution and termination of the tegmental afferents in the interpeduncular nucleus (IP) were studied in the mouse by means of the rapid Golgi method. Primarily on the basis of terminal branching patterns and distribution areas, two types of afferents were distinguished. The type 1 fibers distribute mainly within the rostral half in the form of numerous glomerular endings, the size of which corresponds well with that of the tufted terminal dendrites of the IP neurons. On the other hand, the caudal half of the IP has far fewer fibers than the rostral IP and is innervated by the type 2 fibers, which follow a tortuous course, terminating in dense fiber plexus. Thus, the rostral and caudal IP are innervated in a different fashion by different afferents originating from tegmental regions. These results are discussed in relation to the distribution patterns of another conspicuous afferent system of the IP, the fasciculus retroflexus.

Summary: Afferent fibers to the habenular ganglion (HG) were derived mainly from the stria medullaris thalami (SM), which was roughly divided into a dorsal and ventral bundle. In the left ganglion seen at the level of the rostrocaudal middle, the dorsal bundle gave off collaterals to the lateral habenular nucleus (LH) and dorsal subnucleus of the medial habenular nucleus (MH), while the ventral bundle innervated the intermediate and ventral subnuclei of the MH. On the other hand, in the right ganglion at the level of the rostrocaudal middle, the dorsal subnucleus of the MH was innervated by collaterals from the dorsal bundle of the SM, whereas in the intermediate and ventral subnucleus fibers from the ventral bundle were seen. In the left ganglion at the caudal level, the dorsal and ventral bundle extended medially and joined the same bundle of the opposite side to constitute a dorsal and intermediate component of the habenular commissure, respectively. A third component of the HC, a ventral component, was seen to run between the fasciculus retroflexus of both sides. As in the case of the rostral level, the dorsal bundle of the SM emitted collaterals to the LH and dorsal subnucleus of the MH, while the intermediate and ventral subnuclei of the MH were projected upon by collaterals from the ventral bundle of the SM. At the caudal level of the right ganglion, the dorsal bundle gave off collaterals to the dorsal subnucleus of the MH. In contrast, the LH and the intermediate and ventral subnuclei of the MH were innervated by fibers from the ventral bundle. With regard to terminal patterns of the SM, fibers to the MH gave off many short fine branchlets forming the glomerular structures, whereas those to the LH branched out into numerous terminals to form a dense fiber plexus. Thus, the afferent fibers to the HG in the red stingray exhibited a striking left-right asymmetry.The habenular ganglion (HG) has been known to receive afferents from various brain regions. In mammals, terminal patterns of the afferent fibers in the HG was studied in detail using the rapid Golgi method (RamOn y Cajal 1911;Iwahori 1977) as well as the experimental methods (Herkenham and Nauta 1977). The afferent fibers to the selachian HG have been studied by numerous investigators (Edinger 1892;Haller 1898;Ariens Kappers 1906;Sterzi 1909;Ariens Kappers and Carpenter 1911;Johnston 1911;Bergquist 1932;Addens 1945;Farner 1978;Smeets, Nieuwenhuys and Roberts 1983). In these studies, however, the afferent fibers were analyzed mainly in myelin stained sections and minute terminal patterns of the afferents have not been fully elucidated. As was shown in our previous study (Iwahori, Nakamura and Kameda 1991), the neuronal organization of the HG in the red stingray exhibited the striking left-right asymmetry. In order to understand the characteristic features of the structural asymmetry, information on the minute organization of the afferent fibers is needed.Such being the case, a Golgi analysis of the afferent fibers to the HG was attempted.In the prese...

The courses and terminal patterns of the fasciculus retroflexus (FR) in the interpeduncular nucleus (IP) were studied in the mouse, using the rapid Golgi method. Mainly on the basis of the distribution areas and terminal patterns, the FR fibers are divided into two types. The type 1 FR fibers are coarse in contour and take zigzag courses to distribute throughout the entire rostral half and core region of the caudal IP. In contrast, the type 2 fibers are fine, travel caudally along the lateral boundary of the IP and terminate in the lateral regions of the caudal half, forming a dense fiber plexus. The distribution areas of the type 1 and type 2 fibers are clearly differentiated from each other, from the cytoarchitectural as well as the fibroarchitectural viewpoint. Thus, the type 1 and type 2 FR fibers form different fiber systems in the IP. These results are discussed in the light of the known hodological, histochemical and ultrastructural studies.