[Organization of the primary visual system of 2 rodents Arvicola terrestris and Meriones shawi].

Radio-autography and Fink-Heimer technique were performed in two rodents, the nocturnal Meriones shawi (Cricetidae, Gerbillidae) and the subterranean Arvicola terrestris (Arvicolidae) in order to investigate the organization of the primary visual system. In general the whole visual system is poorly developed in Arvicola. In two species, contralaterally, visual inputs project to nuclei geniculatus dorsalis and ventralis, pretectum and colliculus anterior (at the level of stratum opticum and the stratum griseum superficiale). In Meriones the nucleus geniculatus dorsalis receive specially an important projection. Ipsilaterally, in Meriones, retinofugal fibres reach the two geniculates, the pretectum and anterior third of the colliculus; in Arvicola, fibres do not pass the pretectum where they project to the nucleus tractus optici only. Few differences exist between rat and Arvicola accessory optic systems. In Meriones the system is entirely contralateral, the nucleus terminalis medialis receive one of the two fasciculi superior only. The retino-hypothalamic system in Meriones shawi shows crossed projections and each pathway reaches the antero-medial hypothalamic area. In Arvicola terrestris fibres reach the contralateral suprachiasmatic nucleus; such a structural difference may be related to the behavioural difference existing between the two rodents.