Distribution of neurons projecting to the retina of Caiman crocodilus.

Horseradish peroxidase (HRP) was injected into the vitreous of the eye, the orbital cavity, or the optic tectum of Caiman crocodilus. Following intravitreal injections, retrograde transport of the enzyme was observed bilaterally, but predominantly contralaterally, in a large oblong field of cells at the isthmic level of the midbrain, bounded medially by the trochlear nucleus and laterally by the nucleus isthmi. Control injections of HRP into the orbital cavity and eye muscles labelled motoneurons of the extraocular muscles but not cells of this isthmic field. The field is therefore the source of a projection efferent to the retina in Caiman. Injections of HRP into the dorsal optic tectum produced a fine pattern of anterograde labelling in fibers projecting to the region in which retinopetal cells were identified, but no retrograde labelling of cells in this area. In contrast, the nucleus isthmi showed dense anterograde labelling of fibers and terminals as well as retrograde labelling of cells following tectal injections. These results extend recent evidence for a close evolutionary relationship between the order Crocodilia and modern birds, since birds also possess a well-developed retinopetal system derived from a cell group in the isthmic midbrain.