Peculiarity in the mode of entrance of the optic nerve into the eyeball in some rodents

In the majority of animals the optic nerve enters the eyeball as a round compact bundle of nerve fibers and the optic disc is circular in outline or nearly so. While removing the eyes from the woodchuck and prairie dog for histological material, I observed that here there was an exception to the general rule in that the optic nerve became transformed into a band flattened anteroposteriorly before it entered the eyeball, and that the disc was linear instead of circular in outline. In these animals the disc consists of a long slender band, bending slightly dorsally in the center and gently widening at the ends. The absence of rods and cones along this band indicates that it is a true blind spot. As far as the investigation has been carried on, this form of optic disc was found to reach its greatest development in the prairie dog and has only been observed in members of the family Sciuridæ. Further investigation is necessary to show whether this peculiarity is limited to species of this family.

Johnson 1 has pointed out the presence of an elongated disc in the squirrel and marmot, but he makes no statement regarding the mode of entrance of the optic nerve into the bulbus oculi. As stated above, the optic nerve spreads out in an antero-posterior direction shortly after it enters the orbit and penetrates the coats of the eye in the region marked by the optic disc. In the wood-chuck and prairie dog the flattened portion of the optic nerve is slightly concave on both the dorsal and ventral sides, and the latter is marked by a slight groove extending along the long axis of the nerve midway between the ends of the disc.