Horseradish peroxidase was injected into the superior colliculus or pretectum or both in order to label, by retrograde axoplasmic transport, the retinal ganglion whose cells axons innervate the dorsal midbrain. The dendrites of ganglion cells were sufficiently well-labelled to reveal their overall morphological characteristics. It was therefore possible to compare the number and form of ganglion cells projecting to the midbrain with the total population of ganglion cells as revealed by optic nerve injections, and with ganglion cells labelled by injections in the lateral geniculate nucleus. We found that not more than 10% of all retinal ganglion cells project to the superior colliculus in the macaque monkey. This percentage varies little over the retina, being about 6% of all ganglion cells near the fovea and increasing slightly with eccentricity. The superior colliculus does not receive a projection from P?? cells and only a few P?? cells terminate there. The majority of cells which project to the superior colliculus have a small- to medium-sized cell body and sparsely branched dendritic tree. We have called them P?? and p?? cells by analogy with the gamma cells and epsilon cells in the cat's retina. Anatomically the P?? and P?? cells are heterogeneous, which would be consistent with the physiological heterogeneity found for ganglion cells which project to the midbrain in monkeys. ?? 1984.
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