The decussation of the retinothalamic pathway in the cat, with a note on the major meridians of the cat's eye.

We have studied the naso-temporal division of the retinothalamic pathway of the cat by making large unilateral injections of horseradish peroxidase into the lateral geniculate nucleus. In confirmation of previous work, our retinal whole-mounts show a distinct vertical decussation line separating the contralaterally projecting nasal retina from the ipsilaterally projecting temporal retina. The ipsilateral decussation line is quite sharp, while the contralateral decussation is somewhat more diffuse, with numbers of large cells extending a few degrees into the temporal retina. However, in contrast to the results of optic tract section, our material (demonstrating the thalamic component only) does not reveal any significant population of contralaterally projecting small cells across most of the temporal retina. The previous observation of approximately 200 micrometer of naso-temporal overlap in the area centralis is confirmed here, and evidence is presented that this overlap may increase at eccentricities above the horizontal meridian. Taken together with previously published data, this demonstration of the vertical decussation line has allowed us to estimate the relative inclinations of the major meridians of the cat's eye.