Entrance of Optic Nerve into Eyeball in Rodents. 5 



optic nerve became transformed into a band flattened antero- 

 posterior^ 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 Sciuridse. Fur- 

 ther 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. The optic 

 nerve from a short distance within the orbit to the chiasma has the 

 usual cylindrical outline. This tendency of the nerve to bifurcate 

 is also seen in the squirrel and chipmunk but it has not developed 

 to the same extent as in the animals above mentioned. 



In the rabbit's eye the optic nerve enters the eyeball as a 

 cylindrical bundle of nerve fibers, from which one might expect 

 the optic disc to be circular in outline, but this is not the case 

 as has been observed by Johnson and figured by Haab. 2 The optic 

 papilla is nearly circular but instead of the fibers radiating from it 

 to the different parts of the retina they divide into two nearly 

 equal portions which run in opposite directions toward the equator 

 of the eyeball. From these two bands the fibers spread out to the 

 different parts of the retina. The expansion of the optic nerve 

 on the inside of the bulbus oculi of the rabbit, in contrast to its 

 spreading out on the outside of the bulbus in some forms of the 



1 Johnson, Phil. Trans. Roy. Soc, 1901, 194 B, p. 30. 



2 Haab, cited by Fuchs, " Physiologisches Praktikum fur Mediziner," p. 227. 



