Efferent tectal pathways in two chondrichthyans, the shark Scyliorhinus canicula and the ray Raja clavata.

The efferent connections of the tectum mesencephali in the shark Scyliorhinus canicula and the ray Raja clavata have been studied by using the silver impregnation methods. of Nauta-Gygax ('54) and Fink-Heimer ('67). After a unilateral lesion made through all six tectal layers, three distinct pathways could be observed: 1) an ascending projection both ipsi- and contralateral to the pretectal area, the dorsomedial region of the thalamus, and the lateral geniculate body, 2) a commisural projection to the contralateral tectum and intercollicular nucleus, and 3) a descending projection to the rhombencephalic reticular formation. The last mentioned tract can be subdivided into (a) the ipsilateral tractus tectobulbaris ventralis and intermedius, giving off fibers to the intercollicular nucleus, the nucleus reticularis isthmi, and the medial and median reticular formation of the rhombencephalon and (b) the contralateral tractus tectobulbaris dorsalis, which connects the tectum with the contralateral medial reticular formation. Contrary to what has been found in other vertebrates there is no distinct segregation with respect to laterality of tectoreticular connections. Neither an ipsilateral projection to the nucleus isthmi nor a direct tectospinal pathway could be demonstrated with the techniques used.