The normal cytology and connections of the mesencephalic nucleus isthmi were examined in cell and fiber preparations of the brains of seven avian forms which included those of chicken, guinea hen, duck, ibis, turkey buzzard, pheasant and sparrow. In all these forms the isthmi complex consisted of three parts which appeared to be associated by position, connections and function. The magnocellular part begins at the region of junction of the diencephalon and the mesencephalon and lies ventral to the nucleus mesencephalicus lateralis pars dorsalis. The parvocellular portion of the nucleus isthmi is the largest subdivision of the complex. It appears and disappears within the rostrocaudal extent of the magnocellular portion of the nucleus. The third, most posterior, and most diminutive division of the nucleus isthmi is the nucleus semilunaris. The above description excludes the nucleus isthmo-opticus as a representative of the nucleus isthmi. Connections of the isthmi complex, as assayed from normal and experimental methods, include the lateral lemniscus, the tectoisthmal, the isthmotectal, the isthmocerebellar, the isthmo-oculomotor and the isthmostriatal pathways.Anatomicophysiologic investigation of the nucleus isthmi further supports its homology with scattered reticular cells along the lateral lemniscus in mammals rather than with the medial geniculate nucleus of mammals. The presence of the avian hippus might provide a useful investigative tool.
METHODS AND MATERIALS
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