Quantitative study of the decussating optic axons in the pony, cow, sheep, and pig.

A quantitative study of optic axons in the optic tracts of several species was performed to determine the number and percentage of optic axons that decussate. Animals were prepared by unilateral ocular enucleation, and light microscopic techniques were employed to count the fibers. The mean number of fibers projecting from a single eye was 732,119 for the pony, 1,041,739 for the cow, 534,755 for the sheep, and 442,629 for the pig. The mean degree of crossover at the chiasm was 80.8% in ponies, 82.9% in cattle, 88.9% in sheep, and 87.8% in pigs.