Projection of optic fibers to visual centers in a turtle (Emys orbicularis)

Studies of retinal projections to the thalamus and midbrain of the turtle were based on a personal modification of the Nauta-Laidlaw technique (modified Nauta method) after unilateral enucleation. Decussation of optic fibers in the chiasma is incomplete. In the thalamus, optic fibers are found to terminate in three nuclei – with greater density in the corpus geniculatum laterale (lateral geniculate body) (LGB) and more sparsely in the nucleus suprapeduncular and nucleus ovalis. Retinal projections to the LGB assume a focal pattern, being somewhat more compact in the lateral neuropil region. Optic fibers are also shown to end in a group of pretectal nuclei: n. pretectalis dorsalis, n. lentiformis mesencephalis, n. comissurae posterior. In addition, terminations of optic fibers have been revealed in the three upper layers of the tectum. Peculiarities of preterminal and terminal degeneration of retinal fibers have been distinguished in the different tectal layers. In the second stratum, terminations of large fibers are mostly seen with a characteristic appearance of lumpy pericellular (preterminal) degeneration. In the third stratum, both large and finer fibers degenerate, showing fine debris of preterminal degeneration. Different patterns of terminal degeneration have been revealed in tectum and thalamus. The maximal size of optic fibers in the tectum proves to be larger than in thalamus. Available evidence is discussed with particular reference to comparison of the phylogenetically more recent retinothalamic system and more ancient retinotectal system.     

An autoradiographic study of the efferent connections of the superior colliculus in the cat

The differential projections of the three main cellular strata of the superior colliculus have been examined in the cat by the autoradiographic method. The stratum griseum superficiale projects caudally to the parabigeminal nucleus and rostrally to several known visual centers: the nucleus of the optic tract and the olivary pretectal nucleus in the pretectum; the deepest C laminae of the dorsal lateral geniculate nucleus; the large-celled part of the ventral lateral geniculate nucleus; the posteromedial, large-celled part of the lateral posterior nucleus of the thalamus. Several of these projections are topographically organized. The stratum griseum profundum gives rise to most of the descending projections of the superior colliculus. Ipsilateral projections pass to both the dorsolateral and lateral divisions of the pontine nuclei, the cuneiform nucleus, and the raphe nuclei, and to extensive parts of the brainstem reticular formation: the tegmental reticular nucleus, and the paralemniscal, lateral, magnocellular, and gigantocellular tegmental fields. Contralateral projections descending in the predorsal bundle pass to the medial parts of the tegmental reticular nucleus and of some of the tegmental fields, the dorsal part of the medial accessory nucleus of the inferior olivary complex, and to the ventral horn of the cervical spinal cord. Ascending projections of the stratum griseum profundum terminate in several nuclei of the pretectum, the magnocellular nucleus of the medial geniculate complex and several intralaminar nuclei of the thalamus, and in the fields of Forel and zona incerta in the subthalamus. The strata grisea profundum and intermediale each have projections to homotopic areas of the contralateral superior colliculus, to the pretectum, and to the central lateral and suprageniculate nuclei of the thalamus. However, the stratum griseum intermediale has few or no descending projections.     

Retinal projections in adult and newborn grey squirrels

Retinal projections in newborn squirrels were compared to those in adults by using horseradish peroxidase (HRP) as a highly sensitive anterograde tracer. In both newborn and adult squirrels, the HRP reaction product was found in the dorsal lateral geniculate nucleus, the superior colliculus, the pretectal nuclei, and the nuclei of the accessory optic tract. Thus, newborn squirrels have retinal input to most or all structures normally innervated in the adult. However, the pattern of terminations differed in the newborn from that in the adult, and this was especially apparent in the dorsal lateral geniculate nucleus and the superior colliculus. In the dorsal lateral geniculate nucleus, the regions of ipsilateral and contralateral retinal inputs were clearly less segregated than in adults, although the adult laminar pattern of retinal terminations was partially apparent, even though there was yet no cytoarchitectural evidence of the adult lamination pattern. In the superior colliculus, a marked difference was seen in the pattern of ipsilateral retinal terminations. In the adult, ipsilateral retinotectal input was restricted to a narrow, dense, patchy, mediolateral band in stratum opticum in the rostral colliculus. In the newborn, the ipsilateral retinotectal input was less dense, free of patches, spread in thickness to include much of the stratum opticum and the superficial grey, and spread in extent to include all but the caudal pole of the colliculus. These observations are consistent with the prevailing view that visual connections are initially widespread and become restricted during the course of development.     

Retinal projections and retinal ganglion cell distribution patterns in a sturgeon (Acipenser transmontanus), a non-teleost actinopterygian fish

Retinal projections in a sturgeon were studied by injecting biocytin or HRP into the optic nerve. The target areas are the preoptic area, thalamus, area pretectalis, nucleus of posterior commissure, optic tectum, and nuclei of the accessory optic tract. Furthermore, a few labeled fibers and terminals were found in a ventrolateral area of the caudal telencephalon. All retinal projections are bilateral, although contralateral projections were more heavily labeled. Retrogradely labeled neurons were found in the ventral thalamus bilaterally. Retinal projections in sturgeons are similar to those of other non-teleost actinopterygians and chondrichthyans (sharks), in terms of the targets and extent of bilateral projections. Distribution patterns of ganglion cells in the retina were examined in Nissl-stained retinal whole mount preparations. The highest density areas were found in the temporal and nasal retinas, and a dense band of ganglion cells was observed along the horizontal axis between the nasal and temporal areas of highest density. The density of ganglion cells in the dorsal retina is the lowest. The total number of ganglion cells was estimated to be about 5 x 10(4) in a retina. The existence of a low density area in the dorsal retina suggests reduced visual acuity in the ventral visual field.     

Retinofugal and retinopetal projections in the cichlid fish Astronotus ocellatus

The retinofugal and retinopetal projections of the cichlid fish Astronotus ocellatus were studied by applying cobaltous-lysine to the optic nerve. Retinal axons terminate bilaterally in a preoptic-suprachiasmatic region between the base of the third ventricle and the anterior genu of the horizontal commissure and among periventricular cells along the sides of the ventricle. Other retinal axons innervate the tuberal region of the hypothalamus anterior to the infundibulum. Targets innervated in the pretectum include nucleus lateralis geniculatus and dorsal, medial, and ventral pretectal nuclei. Three other targets (nucleus opticus dorsolateralis, nucleus opticus commissurae posterior, nucleus opticus ventrolateralis) are innervated by fibers that leave the medial edge of the dorsal optic tract. Two other targets (basal optic nucleus and accessory optic nucleus) are innervated by fibers from the ventral optic tract. These retinal projections are similar to those previously reported for goldfish in an experiment that used the cobaltous-lysine method (Springer and Gaffney, J. Comp. Neurol. 203:401-424, '81). Retinotectal optic axons were found in a superficial lamina just above the stratum opticum, in the stratum opticum, in three layers of the stratum fibrosum et griseum superficiale, in a lamina just beneath the stratum fibrosum et griseum superficiale, and in the stratum album centrale just above the stratum periventriculare. This result is similar to that previously reported for goldfish; however, the spatial relationships between the various retinorecipient laminae differ for goldfish and Astronotus ocellatus. Efferents to the retina originate in two nuclei and both project contralaterally. The first is the nucleus olfactoretinalis, which is located ventrally between the olfactory lobe and telencephalon. It consists of about 400 cells, of which, approximately 200 cells project to the retina. The second retinopetal nucleus, nucleus thalamoretinalis, is a diffuse group of about 200 cells that project to the retina.     

Retinal Projections to the Pretectum, Accessory Optic System and Superior Colliculus in Pigmented and Albino Ferrets

Retinal projections to the pretectal nuclei, accessory optic system and superior colliculus in pigmented and albino ferrets were studied using anterograde tracing techniques. Both Nissl- and myelin-stained material was used to identify the pretectal nuclei, nuclei of the accessory optic system and the layers of the superior colliculus. Following monocular injection of either horseradish peroxidase or rhodamine-B-isothiocyanate, four pretectal nuclei, including the nucleus of the optic tract, posterior pretectal nucleus, anterior pretectal nucleus and the olivary pretectal nucleus, could be identified to receive direct retinal input in both pigmented and albino strains. In the accessory optic system, retinal terminals were observed in the dorsal, lateral and medial terminal nuclei as well as in the interstitial nucleus of the superior fasciculus, posterior fibres. The retinal projection to the superior colliculus was found to innervate the three superficial layers. The retinal projections to the pretectal nuclei and nuclei of the accessory optic system in the pigmented animals were bilateral, although the label was most dense contralateral to the injected eye. Ipsilateral retinal projections to the pretectal nuclei and nuclei of the accessory optic system appeared to be absent in albino ferrets, i.e. they were invisible with our methods. In both pigmented and albino ferrets retinal terminals in the contralateral superior colliculus densely innervated the three superficial layers. In both strains the ipsilateral projection appeared as clusters which were absent in rostral and caudal poles. In pigmented animals the ipsilateral projection was much denser and more extensive than in albinos. Following injection of retrograde tracers into the brainstem at the level of the dorsal cap of the inferior olive, retrogradely labelled neurons in the pretectum were found in the ipsilateral nucleus of the optic tract. Their somata overlapped mainly with scattered retinal terminals close to the pretectal surface and rarely or not all with the deeper prominent terminal clusters. In the accessory optic system, inferior olive projecting neurons were observed in all four ipsilateral nuclei and fully coincided with the retino-recipient zones. In the superior colliculus, retrogradely labelled neurons were found contralateral to the injection site in the deep layers.     

Retinal projections in lamprey (Lampetra fluviatilis)

Visual projections in lamprey were investigated using two methods,--one by revealing transport of horseradish peroxidase, and the other by silver impregnation of degenerating axons and terminals after enucleation of the eye. Both methods produced similar results. The chiasm showed incomplete crossing of retinal fibres, the major part of which, as an optic tract, proceed along the contralateral thalamus up to the entry into the optic tectum, while the smaller part takes the same course on the ipsilateral side. Besides, from the posterior part of the optic chiasm an axial optic tract branches off, which proceeds through the central part of the contralateral thalamus up to the pretectal nucleus, individual fibres of which enter the central grey layer of the optic tectum. On the contralateral side, the visual projections are localized in the lateral geniculate body, pretectal nucleus, in the three upper layers of the optic tectum, in the ventrolateral area of the optic tectum and as solitary diffuse projections in the mesencephalic tegmentum. Innervation of thalamic and pretectal nuclei are realized by two tracts--the tractus opticus proper, and tractus opticus axialis. On the ipsilateral side visual projections, excepting the optic tract, are scarce and in the thalamus appear as small areas of the lateral geniculate body and pretectum adjacent to the optic tract. Solitary visual projections were found in two upper layers of the rostral optic tectum and in larger numbers in the 3rd and 4th layers of the caudal part and in ventrolateral area of the optic tectum. Projections in mesencephalic tegmentum were single. Diffuse visual projections in the lateral part of hypothalamus could be revealed only by the silver impregnation method. Using the peroxidase method two types of cells were observed in mesencephalic tegmentum where, possibly, the centrifugal fibres proceeding to retina, originate. A comparison is made of central visual projections in lampreys and other representatives of nonmammalian vertebrates.     

A radioautographic study of retinal projections in Type I and Type II lizards

The retinofugal projections of 5 species (Acanthodactylus boskianus, Scincus scincus, Tarentola mauritanica, Uromastix acanthinurus and Zonosaurus ornatus) belonging to 5 different families of Type I and Type II lizards have been examined by means of the radioautographic method. In the 5 species the retinal ganglion cells project to the contralateral hypothalamus (nucleus suprachiasmaticus), thalamus (nucleus geniculatus lateralis pars ventralis, nucleus geniculatus lateralis pars dorsalis), pretectum (nuclei lentiformis mesencephali, geniculatus pretectalis, postero-dorsalis griseus tectalis), tectum opticum (layer 2 to layer 6 of the stratum griseum et fibrosum superficiale) and tegmentum mesencephali (nucleus opticus tegmenti). Ipsilateral optic fibers were never observed in Uromastix acanthinurus, whereas an uncrossed quota was visible in both nucleus geniculatus lateralis pars dorsalis and nucleus postero-dorsalis in the other species. An ipsilateral retinotectal projection was observed only in Tarentola mauritanica. With the exception of the nucleus griseus tectalis the contralateral optic centers identified in this material have to a large extent been observed in other reptiles belonging to the different orders. The presence in reptiles of a general pattern of contralateral visual projections indicates that these were established very clearly in the course of evolution. Similarities become apparent when this plan is compared with that observed in birds. In marked contrast the ipsilateral component in reptiles is unstable and mutable in nature. This ipsilateral retinotectal projections do not appear to be a feature restricted to Type I lizards. On the other hand, the presence of this optic component cannot be linked solely to nocturnal habits.     

Retinofugal and retinopetal projections in the green sunfish, Lepomis cyanellus

The retinofugal and retinopetal connections in the green sunfish were studied by autoradiographic and horseradish peroxidase methods. All retinofugal fibers decussate in the optic chiasm. Some fibers project to contralateral preoptic and hypothalamic nuclei while others recross to project to the comparable ipsilateral nuclei. Contralaterally, the medial optic tract projects to the periventricular thalamic and pretectal nuclei and, sparsely, to the rostral optic tectum. The dorsal optic tract projects to the parvocellular portion of the superficial pretectal nucleus, the central pretectal nucleus, nucleus corticalis, and the rostral portion of the optic tectum. The ventral optic tract primarily projects to the caudal portion of the optic tectum, giving off fibers in route to innervate various nuclei, including the parvocellular superficial pretectal nucleus and the dorsal and ventral accessory optic nuclei. The axial optic tract projects to the dorsal accessory optic nucleus, the central pretectal nucleus, and the caudal optic tectum. Retinal fibers reach the ipsilateral thalamus, pretectum and other sites via a redecussation through the posterior commissure. From outgroup analysis it is concluded that such redecussating fibers are an independently derived character within actinopterygians and are homoplasous to nondecussating ipsilateral retinal projections in other vertebrates. Neurons retrogradely labeled with horseradish peroxidase were found to form a rostrocaudal column from the olfactory bulb and nerve through the ventral telencephalon to caudal diencephalic levels along the medial aspect of the optic tract. It is possible that all these neurons consist of one population of migrated ganglion cells of the nervus terminalis.     

Tectal efferents in the banded water snake,Natrix sipedon

Visual information reaches the dorsal thalamus by two distinct routes in most reptiles. Retinal efferents terminate directly in the dorsal lateral geniculate nucleus (DLGN). Retinal information is also channeled indirectly through the tectum to nucleus rotundus. Retinal projections to DLGN and tectum are also well esablished in snakes, but the status of the tecto-rotundal link of the indirect visual pathway is uncertain. Thus, tectal efferents were studied with Fink-Heimer methods in banded water snakes (Natrix sipedon). The tectum gives rise to crossed and uncrossed projections to the brainstem reticular formation. Commissural connections are effected with the contralateral tectum via the tectal and osterior commissures. tectum projects densely to the ipsilateral basal optic nucleus. Bilateral ascending projections reach the pretectal area, nucleus lentiformis mesencephali, lateral habenular nuclei, and posterodorsal nuclei. Ascending projections reach the ventral lateral geniculate and suprapeduncular nuclei. there is a diffuse projection to the central part of the caudal thalamus and a dense, bilaternal projection to the DLGN. These results indicate that the relation of the tectum to the dorsal thalamus is different in snakes than in other reptiles. Nucleus rotundus is either absent or poorly differentiated and there is a strong convergence of the direct and indirect visual pathways at DLGN.     

Evidence for cerebellar efferents to the ventral lateral geniculate nucleus and the lateral terminal nucleus of the accessory optic system in the rabbit. A morphological study with comments on the organizational features of visuo-oculomotor-trunco-cerebellar loops

The efferent ascending connections of the cerebellar nuclei and afferent optic projections to the ventral lateral geniculate nucleus and the terminal nuclei of the accessory optic tract were traced in the 26 rabbits using the technique of experimental anterograde degeneration. Following eyeball enucleation, within the ventral lateral geniculate nucleus terminal degeneration was found mostly contralaterally and was restricted to both the sublayers (external and internal) of the lateral division, while ipsilaterally only scanty and confined to the dorsal region of the external sublayer of the lateral (sector alpha) division. After cerebellar lesions degeneration was found within the ventral region of the medial division (sector gamma) of the contralateral LGv and within contralateral LTN. From the localization of the lesions in the cerebellar nuclei, as well as from the distribution of degenerations in the area of the LGv, it was postulated that the parent neurons for the cerebello-LGV fibers are located in the contralateral posterior interposed nucleus, although the anteroventral lateral cerebellar nucleus, the Y group and the infracerebellar nucleus have been not excluded. Within the all terminal nuclei of the accessory optic tract the retinal fibers were found to terminate bilaterally with contralateral preponderance, mostly in the MTN, while ipsilateral fibers terminate most extensively in the lateral terminal nucleus of the accessory optic tract (LTN). In this means the retinal afferents of both sides seem to subserve the contralateral lateral cerebellar nucleus control. Taken together, the findings indicate that the extrageniculate visual inputs might be subjected to direct reciprocal cerebello-nuclear control. The visual extrageniculate cerebellopetal pathways and their correlations with the vestibulo-ocular and optokinetic reflex loops are discussed.     

Retinopretectal and accessory optic projections of normal mice and the OKN-defective mutant mice beige, beige-J, and pearl

Retinal projections to the pretectal and terminal accessory optic nuclei were studied in normal wild-type mice and mutant mice with abnormal optokinetic nystagmus (OKN, Mangini, Vanable, Williams, and Pinto: J. Comp. Neurol. 241:191-209, '85). The mutants used were pearl, which exhibits an inverted OKN in response to stimulation of only the temporal retina, and beige and beige-J, which show inverted OKN in response to stimulation of only the temporal retina and, in addition, exhibit eye movements with a vertical component in response to horizontally moving, full-field stimuli. These projections were studied following intraocular injections of 3H-proline or horseradish peroxidase (HRP) with, respectively, light microscopic autoradiography or HRP histochemistry. In wild-type mice, strong contralateral retinal projections covered the entire nucleus of the optic tract, the anterior and posterior divisions of the olivary pretectal nucleus, and the posterior pretectal nucleus. Similar heavy contralateral projections were distributed over the dorsal and medial terminal nuclei of the accessory optic system. Also, terminals sparsely covered the entire neuropil of the contralateral lateral terminal nucleus in some but not all wild-type mice. The most prominent accessory optic input was to the medial terminal nucleus and was provided by the inferior fasciculus of the accessory optic tract. A typical mammalian superior fasciculus of the accessory optic system with anterior, middle, and posterior components was present. Ipsilateral label was found in anterior and posterior olivary pretectal nuclei in all of the wild-type animals, but was found inconsistently in the ipsilateral terminal accessory optic nuclei. The pattern of contralateral retinal projection to the nucleus of the optic tract and posterior pretectal nucleus in mutants was indistinguishable from that seen in the normal wild-type mice. However, retinal inputs to the ipsilateral anterior and posterior olivary pretectal nuclei were significantly reduced in pearl mutants and were exceedingly sparse in the beige and beige-J mutant mice, while the contralateral inputs to these nuclei were increased in a complementary fashion in the mutants. The labeling of the accessory optic input to the contralateral dorsal terminal nucleus appeared to be substantially reduced in all of the mutant mice. The size of the principal accessory optic fascicle, the inferior fasciculus, was significantly smaller in beige, beige-J, and pearl mice; this reduction was greater in the beige and beige-J than in the pearl mice.(ABSTRACT TRUNCATED AT 400 WORDS)     

Retinal projections in two Australian polyprotodont marsupials: kowari, Dasyuroides byrnei, and fat-tailed dunnart, Sminthopsis crassicaudata (Dasyuridae)

Retinal projections were examined in two small dasyurids, the kowari and the fat-tailed dunnart, following injections of 3H-proline into one eye. In both animals retinal fibres terminate in the dorsal and ventral lateral geniculate nuclei (LGd, LGv), the lateral posterior nuclear complex, the pretectum, the superior colliculus, the suprachiasmatic nucleus of the hypothalamus and the nuclei of the accessory optic system. The lateroposterior thalamic complex and the accessory optic nuclei receive projections from the contralateral eye only; the remaining centres receive bilateral inputs. Both LGd contain an undifferentiated beta or medial segment and an alpha or lateral segment that comprises further cellular sublaminae, 4 in the kowari and 3 in the dunnart. There is substantial overlap of crossed and uncrossed terminals in both segments, though in each animal a narrow cell lamina next to the optic tract receives only crossed projections and the lateral part of the beta segment receives only uncrossed projections. There is a cell-sparse zone within the alpha segment that receives a predominately uncrossed projection in the kowari and a crossed projection in the dunnart. In both marsupials the density of crossed and uncrossed terminals is equal, a feature of dasyurid quolls but not of another dasyurid, the Tasmanian devil. Additionally, retinal terminals do not form dense clusters within the LGd neuropil. This feature is characteristic of quolls, but not of other mammals, marsupial or placental, all of which display LGd terminal clusters. These findings suggest that the functional organisation of the LGd in these dasyurids may differ from that found in other marsupials.     

An autoradiographic study of the projections from the lateral geniculate body of the rat.

The projections from the lateral geniculate body of the rat were followed using the technique of autoradiography after injections of [3H] proline into the dorsal and/or ventral nuclei of this diencephalic structure. Autoradiographs were prepared from either frozen or paraffin coronal sections through the rat brain. The dorsal nucleus of the lateral geniculate projected via the optic radiation to area 17 of the cerebral cortex. There was also a slight extension of label into the zones of transition between areas 17, 18 and 18a. The distribution of silver grains in the various layers of the cerebral cortex was analyzed quantitatively and showed a major peak of labeling in layer IV with minor peaks in outer layer I and the upper half and lowest part of layer VI. The significance of these peaks is discussed in respect to the distribution of geniculocortical terminals in other mammalian species. The ventral nucleus of the lateral geniculate body had 5 major projections to brain stem structures both ipsilateral and contralateral to the injected nucleus. There were two dorsomedial projections: (1) a projection to the superior colliculus which terminated mainly in the medial third of the stratum opticum, and (2) a large projection via the superior thalamic radiation which terminated in the ipsilateral pretectal area; a continuation of this projection passed through the posterior commissure to attain the contralateral pretectal area. The three ventromedial projections involved: (1) a geniculopontine tract which coursed through the basis pedunculi and the lateral lemniscus to terminate in the dorsomedial and dorsolateral parts of the pons after giving terminals to the lateral terminal nucleus of the accessory optic tract, (2) a projection via Meynert's commissure to the suprachiasmatic nuclei of both sides of the brain stem as well as to the contralateral ventral lateral geniculate nucleus and lateral terminal nucleus of the accessory optic tract, and (3) a medial projection to the ipsilateral zona incerta. The results obtained in these experiments are contrasted with other data on the rat's central visual connections to illustrate the importance of these connections in many subcortical visual functions.     

Retinal projections in the australian lungfish

Autoradiographic analysis of the primary retinofugal projections in the Australian lungfish reveals contralateral retinal projections to a ventral portion of the periventricular preoptic nucleus, throughout its rostrocaudal extent, and to 4 distinct terminal fields in the thalamus. Only one of these thalamic fields (t4) likely receives dendrites soley from dorsal thalamic neurons. Thalamic terminal field 1 probably receives dendrites from both dorsal and ventral thalamic neurons, and fields 2 and 3 from only ventral thalamic neurons. Contralateral retinofugal fibers terminate in the pretectum and in the superficial and central tectal zones. The central tectal terminal field is restricted to the medial one-third of the tectum. At pretectal levels a contralateral basal optic tract arises from the marginal optic tract and terminates along the lateral edge of the tegmentum, as a series of glomerular puffs, and in the rostral pole of a superficial isthmal nucleus. The Australian lungfish, unlike the African and South American lungfish, possesses ipsilateral retinal projections to all of the nuclei that receive contralateral retinal input.     

Retinal projections in the native cat, Dasyurus viverrinus.

Retinal projections were examined in the native cat, Dasyurus viverrinus using Fink-Heimer material and autoradiography. We found six regions in the brain which receive retinal projections. These are (1) the dorsal lateral geniculate nucleus (2) the ventral lateral geniculate nucleus (3) the lateral posterior nucleus (4) the pretectum (5) the superior colliculus, and (6) the accessory optic system. We did not examine the hypothalamus. The accessory optic system and the lateral posterior nucleus receive a contralateral retinal projection only and the other four regions receive a bilateral retinal projection. There is extensive binocular overlap in the dorsal lateral geniculate nucleus. On the side contralateral to an eye injection of 3H leucine our autoradiographs show four contralateral layers which fill most of the nucleus. Three of these layers, 3, 4 and 5, also receive input from the opsilateral eye. Layer 1 which lies adjacent to the optic tract receives only contralateral retinal input. Layer 2 receives a direct retinal input only from the ipsilateral eye. The ipsilateral projection to the dorsal lateral geniculate nucleus forms a fairly continuous patch which is not divided into separate layers. The ipsilateral retinal input is located in the dorsal part of the lateral geniculate nucleus. The ventral quarter of the nucleus only receives a contralateral retinal input and therefore represents the monocular part of the visual field.     

The subcortical distribution of optic fibers in Saimiri sciureus (squirrel monkey)

1. The optic subcortical connections in a new world monkey (Saimiri sciureus) have been examined with the Nauta and Glass silver impregnation. 2. Terminal degeneration following optic nerve sectioning reveals a laminated dorsal cell mass, which contains separate points of termination for crossed and uncrossed optic fibers. 3. Experimentally it can be shown that the two magno-cellular laminae have separaate connections. The most ventral lamina (lamina 1) receives contralatieral fibers, lamina 2 ipsilateral fibers. The dorsal cell mass in the normal material undivided shows experimentally the following lamination: layers 4 and 6 receive contralateral fibers while layers 3 and 5 receive ipsilateral fibers. 4. Further points of termination for optic impulses are: the nucleus pregeniculatus, the nucleus limitans, and the superior colliculi (stratum poticum). 5. Fibers of the accessory optic system are crossed entirely and terminate in the nucleus of the accessory optic tract.     

The dorsal column nuclear projections to the nucleus ventralis posterior lateralis thalami and the inferior olive in the cat: an autoradiographic study.

This autoradiographic study demonstrates a topical projection of the dorsal column nuclei to the contralateral nucleus ventralis posterior lateralis thalami and the accessory part of the inferior olive. In contrast to earlier anatomical studies the projections of the gracile nucleus and the internal cuneate nucleus proved to be independent and entirely contralateral. Fibers from the gracile nucleus terminate only in the lateral part of the nucleus ventralis posterior lateralis (VPL1) and from the internal cuneate nucleus only in the medial part of this nucleus (VPLm). Projections of the gracile nucleus to the contralateral inferior olive are restricted to the caudal one-third of the medial accessory olive and the ventrolateral part of the dorsal accessory olive. The internal cuneate nucleus is only connected with the dorsomedial part of the rostral two-thrids of the dorsal accessory olive. Our material does not allow conclusions about projections from the dorsal column nuclei to other thalamic nuclei and about rostrocaudal point to point relationships between the dorsal column nuclei and the thalamus or the inferior olive.     

The retinofugal pathways in the primitive African bony fish Polypterus senegalus (Cuvier, 1829)

Retinal projections were studied using Fink-Heimer and radioautographic methods in Polypterus senegalus, a species which is representative of a small group of African fresh-water bony fish often considered to be very primitive.The large optic nerve showed partial decussation at the chiasm. Two major contralateral tracts were observed: the axillary and marginal optic tracts, with the latter being subdivided posteriorly into the tractus opticus medialis and tractus opticus lateralis. The retina projected onto the: (1) hypothalamus (area optica postoptica); (2) thalamus (nucleus opticus dorsolateralis thalimi, nucleus dorsomedialis thalami, corpus geniculatum laterale, area optica dorsolateralis thalami, area optica ventrolateralis thalami); (3) pretectum (nuclei commissurae posterioris, pretectalis ventralis, pretectalis dorsalis); and (4) optic tectum (stratum marginale, stratum opticum, stratum griseum et fibrosum superficiale, stratum griseum et album centrale, stratum griseum et fibrosum periventriculare). Ipsilateral retinal projections were demonstrated to the same 4 levels and more precisely to the nucleus opticus dorsolateralis thalami, area optica dorsolaterale thalami, nucleus commissurae posterioris, stratum marginale and stratum griseum et album centrale. The existence of a retinal projection to the mesencephalic tegmentum is discussed.Comparing the primary optic system of Polypterus with that of other jawed vertebrates, and particularly with that of other bony fish, indicated that this species possesses a combination of characteristics which are both actinopterygian and sarcopterygian. The phylogenetic significance of this mozaic anatomical arrangement is discussed.